Depends on what you mean by 'endgame.' It's entirely possible to complete the main quest of Morrowind by about level 15-20 without doing anything particularly weird, and the game seems to have been designed with the assumption that level 20 is about as high as you'll get in normal gameplay - the leveled lists for shops and loot don't have anything special reserved for characters over level 20 (most of them, for that matter, stop adding 'better' things in the mid- to late-teens; I think grandmaster alchemy equipment might be the only thing that any leveled list reserves for level 20+ characters), there's a late busywork section of the main quest (the House Hortator and Ashlander Tribe Nerevarine quests) that can be skipped if you're already level 21+ and have enough renown by the time you get to it, and to my recollection there's no new creatures spawning in the world past the late-teens or maybe low-twenties. Tribunal and Bloodmoon each probably add about ten levels to that, so endgame Tribunal is probably level ~25-30 while endgame Bloodmoon is probably level ~35-40 - though you could probably manage them at lower levels if you really wanted to do so.
If by 'endgame' you mean something closer to 100% completion of all available faction quest lines, then that'll probably take longer and Hagashager's mid-high twenties / high thirties / mid forties implied estimates for Morrowind / Tribunal / Bloodmoon seem reasonable to me.