It's not long at all - if anything Mooshu is one of the shorter worlds!
You haven't seen anything yet, the worlds just get longer and harder. Compared to Mooshu's 91 main quests & 28 bosses, Dragonspyre has 106 main quests & 35 bosses. Then you hit the "second arc."
Celestia has 95 quests & 26 bosses (and 3 of them cheat). Zafaria has 148 quests and 24 bosses (but 6 of them cheat). Avalon has 161 quests & 43 bosses (4 of them cheat). Azteca has 197 quests & 57 bosses (and 6 of them cheat). The worst world of all is Khrysalis, with 277 quests and 76 boss fights (of which 18 cheat).
So no, Mooshu is not that long at all compared to what's coming.
The "third arc" of worlds are much shorter in terms of quests, thankfully. Polaris has only 99 quests and 39 bosses (but 7 cheaters). Mirage has 113 quests and 60 bosses (and 21 who cheat). Empyrea is the shortest world beyond Mooshu, but it's also only half released. So far there are 79 quests and 24 bosses (with 8 cheating ones). I expect it will have somewhere around 160 total quests when part 2 of Empyrea is released.