Only normal boss girls have an increased chance of dropping the more you play, but it is very, very small, almost non-existent. It also may be affected by Boss level, but can't be sure about this. I do know that Legendary and Even girls have static drop rates that do not change.

I also know that it has been said more than once, that the more girls you have, especially once your harem reaches 100 girls, that your drop chance becomes much, much more scarce.