I try to keep it to a mile or less without spare tube and pump. I'll occasionally go for a long ride only along bus routes, completely unencumbered.

Once in a blue moon, I'll find myself in a situation. The other day, I'd left my bag o' fixin' stuff at work, but had to go get supplies on my way in. Of course the supply store was about 5 miles out of the way and not on a bus route and of course, I got a flat. I started to continue walking in to work 'til I calculated I was 3.5 miles out! No way I'm pushing flatted bike that far. Luckily I was only 1/2 mile from the bus route, but with a transfer required, it took me 45 minutes to get to work from that point.