I remember the year I spent in Icecrown Citadel. I'm not really exaggerating - it was from December to December, so about a year total. It was about the longest time I spent on a raid, including the days of Molten Core - for comparison, Molten Core was the only real endgame raid besides Onyxia's Lair from November of 2004, WoW's release date, until July of 2005, so roughly eight months. Interestingly, the Shadow of the Necropolis patch (patch 1.11) came out in June of 2006, so in the year between the first and last raids of classic WoW we saw MC, Onyxia, BWL, Zul Gurub, Ruins of Ahn'Qiraj, Temple of Ahn'Qiraj and finally Naxxramas. All of these raids released between July of 2005 and June 2006. Not all of these raids were replacements for previous ones - Blackwing Lair and AQ 40 were considered 'sidegrades' from each other, at least until one killed C'thun, who until the release of Naxxramas had the best gear in the game. The two 20 man raids, ZG and AQ20, did not replace BWL or evn MC gear, they just provided another place to go.
Because of the way raids were structured back then it's a little misleading to compare classic's raid release schedule with our modern one. Raids were something a very few players overall did - there was no parity between smaller and larger raid sizes, no LFR, no flex (although by the time Naxxramas came out, some guilds were running MC, Onyxia and even BWL/AQ with smaller raids to maximize gear acquisition before heading into Naxx) and the only way to gear up for raids was either to be carried through said raids by geared groups and handed all the stuff they didn't want or need anymore, or to start on the ground floor and run the level 60 dungeons. The design wasn't structured around raiding being accessible or allowing a larger group of players to see these fights - raiders got to see them, and if that was 10% of the people playing the game, that's what it was.
It's interesting to look at how players react to raid content now. A commonly expressed sentiment is that Throne of Thunder, a raid first released on March 5th, 2013, has been around too long and players are eager for new content. This is a raid that has been around for six month, and will be superseded around the time it enters it's seventh. While hardly the shortest time a raid has ever had to be run through, it's not much longer than the initial tier of Mists raid content, either. Mists of Pandaria released on September 25th, 2012, meaning that from October 2012 to March 5th 2013 we only had MSV, HoF and ToES - a time of about five months. What makes five months acceptable and seven months unacceptable? Are two months that much longer to raid a zone?