I would suggest any time after lv23. At that level Gloom learns Mega Drain which isn't that worth while but it is a move that Bellossom can't learn. At that level Bellossom learns Magical Leaf.
The other move that may be worth waiting for could be Moonlight which Gloom learns at lv41. So if you want Magical Leaf and Moonlight that bad then you could always use a Heart Scale to learn Magical Leaf after evolving once learning Moonlight.
The latest I would suggest to evolve Gloom is lv53 since at that level Bellossom learns Leaf Storm which is powerful considering that Bellossom's special attack is the highest stat is has.
As long as you don't want Giga Drain or Petal Dance on your soon-to-be Bellossom, anytime around level 23 or higher would be fine. Otherwise, you'll have to level Gloom up to 47 for Giga Drain or 53 for Petal Dance.