There are various factors that ought to be considered when making such an incredibly, earth-shatteringly important decision. The biggest consideration is the summon time for a mount, firstly, which requires that your character stand still for 1.5 seconds. That time is time wasted, time which could be spent running towards your destination. And what's more, many characters have speed boosts, like Blink, Heroic Leap, Feline Swiftness, Body and Soul, and so on. Those all have to be factored into your decision: to mount or not to mount?
I posted this question on Twitter last night, and among the discussion, a mathematical force emerged. Jarre, druid blogger at Rank4HealingTouch, leapt in with some calculations. They obtained their numbers for a character's base run speed, and put together what they referred to as "a bit of napkin math".
The "napkin math" was definitely not appropriately named. What Jarre actually put together was the start of a table which indicated a few things, specifically, in various circumstances, and with numbers to back it up, indicating how far you have to be traveling for mounting to be the best option. Jarre has also put in numbers for how long it would take a mounted player, who'd paused to summon their steed, to catch up with a walking player, including one with several different speed boosts.
Check out the table and see whether you should be mounting or running!