We are noobs- only found 10 so far, and have already had a few DNF's. Already, we hate the feeling of a DNF, so I feel like we look for a pretty long time, but it has really depended on several factors.
We are not fans of microcaches, but have tried to find a couple. It seems like we've spent more time on those because we know they are generally going to be tougher. Our first DNF was an urban microcache behind a pizza place. We spent waaaay longer on this one than we should have considering it was a microcache in a location without much relevance. However, we had others with us and it just became one of those times when we were determined to find it for the principle of the thing! It had been found the day before, yet there were five adults, all with their GPS honed in ground zero and we couldn't find it. We looked an hour.
Other times, we've spent less time before logging a DNF. Again, usually because the place just wasn't that interesting or relevant and we had other caches on our list we wanted to try to get to. We've logged a DNF after only 15 or 20 minutes of looking if we really felt like it wasn't there or the clue was not that great or whatever. We usually try to be pretty clear in our log if we do leave a DNF, so that the CO doesn't worry that it's missing (unless we are pretty sure that it really is missing)!