Ok, first things first. If you have any slots that still aren't occupied by epic-quality items, you're probably going to find upgrades for those slots before you leave either of Northrend's two starter zones. If you're rocking The Bringer of Death when you step into Borean Tundra, the first non-druid staff you come across there will absolutely usurp it, and it'll be a large upgrade. Anything pre-Kara, in fact, is likely to become obsolete very early on in the expansion.
In addition, there are certain items --tier 6 pieces or high level PvP gear for example--that may last slightly longer for you because of set-bonuses, unique effects, or gem slots. You may find items that have comparable stats in the expansion, but they don't grant you interruption protection on your Polymorph spell or increase the duration of Evocation for you. Certain high level pieces or sets may last you all the way to 80, just for their difficult-to-replace benefits.
We'll look at a few of the major item slots, to see if we can identify a pattern. I'll try to list list both a low and high-end example of a level 70 epic for that slot, and then tell you what the first likely upgrade is and where you'll find it in the expansion. This should give you some idea, by comparing your own gear to the items we list here, of how long you'll be sporting each item when you plunge into the new content.
Staff of Infinite Mysteries
Tower of the Infinite Mind
Kara gear, as a rule, doesn't last very long in the expansion. It'll be very nice to have going in, and will get you through most if not all of the two starting zones, but not much farther. This staff is a quest reward found near the end of Borean Tundra, and though you lose a bit of stamina and intellect from the Kara staff, you more than make up for it with the extra hit rating, crit rating, and spellpower.
Zhar'doom, Greatstaff of the Devourer
This staff, a reward from the Northrend version of Nagrand's Ring of Blood quest-line, will replace anything the current crop of epics has to offer, with the lone exception of the Golden Staff of the Sindorei, which drops off Kil'jaeden himself in Sunwell Plateau. You can embark upon this quest-line as early as level 75, though a group at that level may struggle to complete the entire chain. This means your high-end epic staff of choice will last you at least five levels, and possibly more.
Keleseth's Blade of Evocation
This drops from the first boss in the first instance in the expansion. Unless they tweak the difficulty before it goes live, this instance should be a cake-walk for a group of reasonably-geared level 70s, so your new 1-hander won't be too tough to obtain.
The Maelstrom's Fury
This is a Blacksmith-created item that can be equipped at level 77. If you happen to have Sunflare--the most powerful one-handed caster weapon in the current game and another Kil'jaeden drop--you won't find an upgrade until you reach level 80.
Wicked Witch's Hat
This is a level 73 quest reward in the Grizzly Hills. It loses you crit rating, but makes up for it with a nice chunk of stamina. If you consider it a more of a side-grade than a true upgrade, you'll still definitely want to look out for a level 75 tailoring item: Duskweave Cowl.
Cowl of the Illidari High Lord
Hat of Wintry Doom
Another tailoring piece (I like using these as opposed to instance drops, simply because they'll be more widely available at the level), this hat is nice, and even includes a replacement meta gem slot. There are other items that may or may not pop up earlier for you, but with all of them, you'll be making the tough choice between the meta gem slot and a few stamina or intellect or something. This, to me, is the first clear upgrade. If you happen to have anything better, like the tier 5 or 6 headpiece, you'll probably be taking it with you all the way to 80.
Mantle of the Mind Flayer
Skein Woven Mantle
This is another Utgarde Keep drop, and though you lose spell penetration (a stat Blizzard has all but phased out of the expansion) and a small amount of spellpower, you gain intellect and a nice bit of mana regen. It isn't the clearest upgrade, I know, but it is an upgrade.
Shoulderpads of Knowledge's Pursuit
There are also a few level 80 5-man instance drops that you may encounter before you actually hit level 80, but this tailoring recipe will probably be your first best chance at an upgrade, and these particular shoulder-pieces can't be equipped before level 80.
So now that we've looked at a few specific slots, let's return to our original question: How long will my gear last me? The pattern we see playing out above repeats itself with very few exceptions throughout each item slot. Low-end epics will be replaced in most cases by level 71 or 72, but can sometimes last as far as the mid-70's. High-end quality epics will carry you quite a bit farther, to level 78ish, and in many cases, all the way to level 80.
I know that item stats haven't been finalized yet in the beta, and with a few tweaks, all of this could change. But for now, forecasting the future as best I can, this seems to be the pattern. Should you continue, in these final months of the Burning Crusade, to grind out gear upgrades for your Mage? I'd say absolutely, positively "yes."
If your Mage is in a progressive raiding guild, continue the push. Sunwell gear will in most cases trump anything you find before the new end-game. Those epics your guild worked so hard to earn will help you gain the next ten levels and only be replaced as you gear up to enter the expansion's first raid.
If PvP is your thing, you won't find much worth ditching that Brutal Gladiator's set for until you get to the level 80 season 5 set. It's possible that you might encounter the odd piece that tempts you to break the set, but we won't see anything similar to what we saw at the beginning of the last expansion. You certainly won't want to drop your high-end purples for anything you'll find in the first few zones of Northrend.
So rest easy. Your epics will have value when Wrath hits. There simply isn't going to be any sort of hard gear reset this time around. Tell any guildies who might be resting on their laurels, waiting for the expansion to hit, that they need to get off their butts and keep pushing. Black Temple gear, Hyjal gear, even Zul'Aman gear, will last you substantially longer than Kara gear, so get moving. Get that raid organized or puff up that Arena rating, so when the expansion comes, you can blow through it like the glass cannons you are.
Every week Arcane Brilliance teleports you inside the wonderful world of Mages and then hurls a Fireball in your face. Check out our recent look at the potential of the new Frostfire Bolt spell, or our analysis of the WotLK beta changes to the Arcane, Frost, and Fire trees. If you're sick and tired of all this Mage-talk, there's a veritable treasure trove of guides and tips related to all of the other aspects of WoW over in the WoW Insider Directory. Until next week, keep the Mage-train a-rollin'.