There is no set time over which a sow will farrow. Some can take literally hours and others pop them out in no time at all. Generally speaking once she lets the piglets latch on and starts grunting as they feed she should have finished farrowing. However, they don't always read the book and and another might appear. The afterbirth may come quite quickly after farrowing or again it might take sometime.
The uterus of a pig is made up of the two horns of uterus. As said never pull the afterbirth away. If you think she is distressed then you could try examining her to see if another piglet is on its way but if you are unsure then please call your vet.
It is very unusual to have a problem farrowing. Only 2% of farrowings present a problem.