Well, she didn't. The only reason Milie accepted the gramps' harassment is because he would harass the cleric girl otherwise. Basically, Milie interceded for Layla's well being. Little did she know that gramps would break his part of the deal and go behind her back. After that, he had 3 full days to corrupt the girls and turn them into their toys. But as you said, aren't 3 days way too little time?
Well, remember the ghost? The blue flame that possesed the old man? He whispered something to him before and after entering his body, right? Maybe the ghost has some kind of power that makes corruption easier. Sadly we will never know, so as the other guy said: enjoy what little we have.