There isn't really an 'ideal' time as such - how long it takes varies from person to person, age, how long it is since they last orgasmed, mood and occasion, how much foreplay there is (and indeed when you start timing, whether you include foreplay), whether it's through sex or masturbation, and goodness knows how many other factors.
I'd guess a typical range is anything from a couple of minutes if you're in the mood for a "quickie", up to maybe 30 minutes (more if you include some extended foreplay).
If you frequently have trouble lasting much more than 2 minutes or so then I'd consider that Premature Ejaculation.
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