What is the max level?
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Depending on the version you're playing (v0.1.2 is the current I believe) when the hints read, "there are no more *girl's name* events in this version," that's usually the sign that that girl's level has been maxed.
Keep in mind the max levels for each character aren't all the same: some may be as high as 10, while others may be as low as 3 or 5.