How long shall a kiss last?

Depends on what kind of kiss, where it is, who it is, and when it is. A first kiss (romantically) should last maybe 3-5 seconds. A kiss on the cheek should be just a peck, maybe 2 seconds max. If it's a first date kind of kiss, I'd suggest maybe a peck that lasts about 2-3 seconds if that's what the mood is, or if you're both really obviously feeling it, maybe a little more than a peck lasting about 4-6 seconds, and it it keeps going, good on you. Now if we're talking a teenage relationship when parents aren't home and a movie is playing, I'd give it a good 1-2 hours with lots of tongue, biting, grabbing, and grinding. All in all, let the moment decide.

Depends entirely on the circumstances. A kiss can be anything from a peck on the lips to full on foreplay lasting upwards of 30 minutes.

If you've been together three months or longer, the kiss should last 12 seconds. This applies if you are at a party. In private both partners can flow with the tide and it can linger on for quite a while as per their passion, urge and desire.

Until her bra is undone.