I think Rose is right…probably between 75k and 100k. If it’s under that it’s always okay to classify it as a novella, and if it’s over it’s just a big book (yay!) or you can probably cut the length down in edits.
Personally I like aiming for 80k (which is usually too short for me XD).
From my reading and experience, 100k is a good average figure! I think the minimum is about 75k, but because it’s a fantasy it could easily go up to 150k, though that might be a bit harder to market.
However, if you’re a little under or over that, it won’t make a whole lot of difference.
I hope this helped!