As far as technical issues are concerned, the MPC gives you plenty of sampling time. If you mean the legal time limit, then make the sample as short as possible.
DJ Shadow said it best when he cited a 4-second limit and using small run vinyl pressings. It depends on the artist. Someone with popular appeal can have enough financial and legal backing to make even a one-second sample impossible to clear. But listen to this..
A few months ago one of the popular music magazines had an article about sampling from the 1970's song "Won't Get Fooled Again" by The Who. Someone had managed to get clearance to sample the entire song! Management either did not know or did not care what could be done. So you never know what might happen.
The challenge is to get the sample clearance. When you find something you want to sample you obviously don't want to wait weeks or months for an approval letter from a lawyer. You want to use the sample right away. If your request gets denied later, then you end up having to remix. Many old recordings on vinyl have been re-issued on CD, so never assume that the vinyl you bought for 50 cents at the used record store is old, used up material that no one wants.