I think the amount of time having fun vs. time studying is relative to each individual.
In my case, I don't "party" too much, but I watch too much TV (which is my way of having fun). I'm one of those people that can cram the night before and still ace an exam, but some of my friends need to study days in advance in order to make a B. At the same time, I know people who can party all weekend, and still have a great GPA.
Just find out what works for you, what kind of GPA you can live with, and what you need to do to get it. Don't miss out on your college years because you want a 4.0 and spend 4 years locked up in the library. I tried this, and only ended up unhappy, cranky, and burned out. Enjoy yourself, do your homework when and how you want to, have fun when you need to, sleep when you need to, and don't overwork yourself. Sure, you might not end up with a 4.0, but you will be much happier and less bitter (like many pre-meds, ugh...)