I use GHS Super Steels or DM Hi-Beams if I'm going to play fingerstyle. I will not put nickelwound strings on my bass unless I'm going to use a pick and I do have to change the Nickel strings much more often.
I can get probably 12-15 hours of playing on steelwound before I start getting bummed about the sound and about 10 hours of playing on Nickel. It's a little longer in the fall/winter because there's less sweat/humidity.
If you play one outside show with a set of strings and it's damp/humid, it will usually kill them IMO. If they're brand new you might get a little more tread out of them but dampness/contamination really takes the zing out.
The rule of thumb is if you keep saying, "man, what happened? why does this not sound good today?" it's probably time to change the strings.