I don't know for a fact what max level is, but I assumed that film categories have max levels of 50, so you can calculate each girl's max by the number of filming categories they have. In fact, Indira and every girl after has 14 categories, so these probably all have a max possible level of 700. The max seems to be 50 in a lot of areas in the game and skills were just updated from max 50 to max 60 during the last major update, but many areas are still at 50, so I assumed film categories were, too.

Once you hit 40 or more in each category, exp mostly drops down to 1 per filming session and less (depending on the filming room), which makes leveling drag out... I understand why it happens, that higher levels need much more filming exp to gain, but it still sucks. Feels like grinding in some mmo again! lol... If you look under the Wiki category for Fake Lay, under Films, the stamina leveling process is sort of explained there, which is linked to the girl's levels. It doesn't mention a girl's max level there or what the max level of each film category is, though.

When I first started playing, and without knowing a lot about the game's mechanics, I was thinking the max level of each girl might be 999, because 4 digits wouldn't fit well on the star that contains each girl's total stamina/level number. Probably stupid, but at the time it made a little sense...