If you are talking about a Shifter, then the answer ia basically in the book: They are animals. End of Story. their natural form is their animal form and they can stay that way forever. They can breed with other animals in their species - but NEVER, EVER with humans. No matter how much they try.
If you are talking about someone under the effects of a spell, well that ends when the spell is no longer sustained (such as by sleeping). If the spell had been quickened, well... then he's got an issue.
Remember, the spell only gives physical characteristics - NOT knowledge. So the poor victim can't communicate with other members of the species he has been turned into, nor with his original species. He can bump uglies with that species all he wants - it won't result in offspring as he is not genetically compatable.