Who it's for, and when
Who it's for: couples who want to make it last, and to see how high pleasure climbs when you let it stretch out. For him, it's a school of control: nearly all young men come fairly quickly (it often improves with age), and learning to feel your climb is how you learn to pace it. For her too: drawing out the arousal phase can lead to a more intense orgasm.
Who with: a partner you know well, who puts no pressure on you to perform. No stopwatch here: the point isn't to last a certain number of minutes, but for you both to be satisfied.
Before: practise solo first, and often, so you don't get discouraged once there are two of you. The more you practise, the less you'll need to stop.
When: rested. Early morning works well, the body is fresh. Late at night, tired or feeling low, is the worst time: give yourselves some rest first. After a long stretch without sex, control is harder too, and that's normal.
Precautions: if old emotions surface, stop and share them. A counsellor or a sex therapist can help, just as they can if coming too quickly persists and weighs on you.
If the erection fails every time, see a doctor first to rule out a physical cause. Pain or discomfort that persists after a session also deserves medical advice. If you're pregnant, have a heart condition or limited mobility, ask a health professional for advice (good to know: her on top is easy on the belly and asks little effort of whoever is underneath).