How long should a guy last in BED?Watch
It's not a rule of thumb, but it should last as long as both parties want it too.
If you're asking as a virgin, there's no definitive answer, but over 10mins is most people's 'acceptance'. Although quality trump's quantity more times than not.
If you're asking within a relationship of sorts; If one party is faster than your desires AND wants to last longer, the source of the issue must be tackled despite this being a embarrassing or uncomfortable process. The quicker this happens the quicker sex lasts longer. If they don't want to go longer, perhaps a re-evaluation of what you both want from eachother and why is a better question.