I think it depends on you. What is hilly to you, might be flat to someone else. That also changes with time.
When I first started commuting I remember calling my commute 'fairly hilly'. It was 5 miles.
I now do a 12 mile each way commute that takes in nearly all of the original commute. I am a lot fitter than I was then, and there is no way I would call my original commute hilly now. My new one is fairly hilly, and contains a few steep sections with 10%+ gradients for reasonable distances. I also have an alternative hillier route!
I've also found that the more I cycle the more I want hills. I even moved house to get closer to them.
