I've just been out and put another 16 miles into my single speed. I decided to have a stab at a bit of climbing, so I did a route with about 700 feet over those 16 miles. That works out at about 44 feet per mile, which is what I've averaged over the course of each of the past 3 years, so that's a fairly representative ride in the grand scheme of things. It was hard work at times, but I got on OK. The worst gradient I was up against was short bursts of 6-7%, and I'm not sure I'd want to do much more than that, or for terribly long, but I got through it OK.
I've planned out a route to our usual cafe, with my single speed in my mind. The cafe is about 250 absolute feet higher than my house, but getting there means about 800 feet of climbing and 550 feet of descent over about 15 miles, so that'll probably be quite a challenge on this bike. I'll have to reward myself with a big cake if I make it.