THREE DAYS.

[EbonVanguard]

Fracture, took around 5-6 hours

[Pinkie Pyro]

Metroid: Zero Mission in 58 minutes.

Sonic the Hedgehog 2: under an hour

[smashbrolink]

I thought this was going to be a Majora's Mask thread, at first.XD

Anyways.....hmm.......
I don't really have an answer for that. I don't time myself.
I do know that most games with a definite end don't last me more than a week tops, though.
If we're talking total play-time, though, then most don't usually last me more than a day or two in total play time before I've beaten them at least once.