The question may be simple to ask but answering it is hard. For starters, when does sex begin and when does it end? Is it:
- from the first kiss to when you fall asleep, or put your clothes back on
- from the moment you think "hey, I’d like to have sex right now” to when you don't want sex anymore
- how long should you spend performing oral sex on your partner (or how long you can expect to receive it)
- How long does penetration last for? And is it vaginal penetration or anal penetration?
Quickly a simple question turns into dozens of other questions, and no answer.
Researchers have tried to look at the “average” amount of time people have sex. But this research either asks people to recall, or asks them to use a stopwatch (literally) while they are having sex. Often this research is done in the context of premature ejaculation, so the framing of the question is usually “how short” rather than “how long”. Much of this research, in my opinion is flawed. First of all, there’s no telling how having a stopwatch in bed with you changes the way you have sex (researchers will sometimes ask couples to literally time their intercourse on a stopwatch). Secondly, studying one relatively flat aspect of sexual activity (length of time) says so little about what’s actually going on in bed.
The best answer I can give you is that good sex lasts for however long the people who are doing it want it to last for. If you only have time for a quickie, then three minute sex is perfect. If you’re on vacation and you’ve got nothing but time, spending all night having different kinds of sex may be what’s ideal.
Also, how long sex lasts very likely changes not only from sexual encounter to sexual encounter, but over time. The sex someone has in their early twenties may be very different than the sex they have in their late thirties.
To address the “all night long” part of your question, it’s worth mentioning that having sex “all night long” doesn’t mean having intercourse all night long. Couples may spend hours in bed together, but usually sex play happens and then there are breaks, and then it happens again.
The idea that people are engaging in one behavior (oral sex, penetration, kissing, etc…) for hours on end without a break isn’t likely. For a more entertaining take on these issues, I recommend the hilarious-because-its-true music video “Ooh Girl! – An Honest R&B Song” playing now on YouTube.