Distance

For every person it is different.

My gf and I made it work when she was doing her study abroad semester in Europe and I was here in the states. It was rough, particularly because I didn't have a schedule conducive to getting there to see her, but we made it through. However, not everyone will, and there are plenty of instances of those who fail while trying.

There are others I know who anything more than a half-hour is too much.

Ultimately, a lot of it is about maturity and how you really value the relationship.

(for what it's worth, she called it quits 9 months after she returned because she couldn't deal with a 4 hour drive when she had an additional semester of school left and I had graduated).