Depends on the girl, and depends on her level of interest. For instance, if she's new to the school or it's the start of a new school year, she'll meet a lot of guys and might have a minicrush on some of them for a day or so apiece, then lose interest. If she gets to know a guy more and they hit off well, she might like him for 1-2, maybe 3 weeks. If it's the guy friend she fell for but he's not interested, she could hang on to that feeling for a long time. It varies. In general, your window of opportunity is 2-8 days.