Well, I had done practically all of my side quests, and I was level 47 (almost 48) when I got the Labyrinth quest. When I was level 43, I was almost done with Mooshu.
If you are going along the main storyline at a steady pace that is comfortable for you, then it doesn't matter what level you are. But I would suggest doing a few side quests every now and then to help you advance even farther. And beware; you need to be level 48 and have defeated Malistaire in order to get to Celestia.