How often should a boyfriend and girlfriend talk?

I have a friend in college who is really good looking and he's a great guy and all, but over the year that I've been around him, he's gone through about 5 or 6 girlfriends (all of the girls broke up with him on their choice). The reasons for this were vague in each case, but I have a feeling it's because my friend might be too clingy/attached to them.

For example, with his most recent girlfriend, he took every single opportunity of free time to go see her, and when we would go somewhere to eat in between classes, he would always bring her along and pay most attention to her and being extremely flirty with her (basically making me feel like a 3rd wheel). And when they weren't with each other, he would text her almost nonstop, talking about such small things, like what they were doing at that exact minute and such.

Basically, my question hopefully to be answered is: If talking with your girlfriend constantly causes break ups, how much time out of the total should be spent talking/hanging out with them?

I'm just curious how much distance should compliment a good relationship.

Thanks to anyone who helps.