What level would they be (i.e. you're playing it as designed) or what level should they be (i.e. how would you like it to be) - that's two different questions. I voted as if the latter question were asked.
It depends entirely on how many encounters the party is having per session and how difficult those encounters are. But, if you follow the guidelines in the DMG closely, you should level up to 2 and 3 in one session each, and approximately every other session thereafter. So, level 20, plus 10 epic boons.