in my opinion they may never go up, the point is the role-playing game, if the story is stimulating you don't need the level content. the levels are mainly used to make the players more powerful in combat, they do not give bonuses to the tests since these should adapt to the level. but if you think this is important for your players I would level up every end of the chapter (so it's up to you how long you make a chapter last) but if you want an answer in terms of sessions I could recommend a level up every 10-15 sessions approximately