Anything that goes up - like skibum says... 100 metres or a few kms...
A low gradient can still be tough if it goes a long while, or if it comes at the end of a long ride...
I'm aiming to stay above 15km per hour (thats about 10miles) and once I acheive that I will aim to stay above 20km (about 12.5miles)
Like I've said before, at the moment, hills are not my strength