For me, time isn't really a factor in itself. It has more to do with what the risk is and what kind of sexual activity is occurring.
Manual stimulation has pretty much no risk as far as STIs are concerned. While rubber gloves are probably a good idea at an orgy, it's probably safe enough to not use them otherwise.
Oral sex is low risk, and most of the infections that can be passed this way are curable. If it's casual sex with someone I just met, I'll use barriers. Although, I don't have casual sex very often. If I can be reasonably sure my partner is low risk, I'll proceed without protection.
Vaginal penetration is a little different. The risk is much higher, the additional concern that this is how babies are made is present, and as much as some men complain about condoms killing the feeling, I've noticed very little difference in feeling with a condom and without. If testing is done when and where relevant, and another form of birth control is in use, I'll consider it.
Anal sex... I only do if it's requested of me, otherwise I really have no desire. There's lots of other fun stuff to do. I always use a condom for this, because it doesn't come up often enough to calculate acceptable risk and it's easier than doing math.
This will most likely change if my sexual behavior becomes more active. In general, "Low risk" with testing when relevant is acceptable, medium risk I'd rather avoid, and high risk I won't do.