I can go over a year between serious hills, since I live in the flatlands. I'm strong on slight upgrades - I really do love the long, gradual hills. For genuine HILLS, though, I have to work to keep it at 6 mph... but don't always succeed. It all depends on the hill, though! (And toting The Limo this weekend - I have no idea what speed I was going 'cause the computer wire is broken, but it wasn't fast, and those weren't really steep hills.)
I do focus *hard* on form... using as many different muscles as I can so it's not all stress on the same ones.