Minecraft – the probability that a villager is a nitwit (green coat)

You will need to return at least 2 villagers. Then they will start repopulating the village. As of 1.2, villagers will mate and repopulate based on the number of available houses. See this post (especially the "your village died out" section) for more information.

As of the 1.8 snapshots, though, the villagers need to be "willing" to breed in order to breed. It seems that you can make a villager "willing" by trading with them. Willingness is granted on the first new trade offer, and at a 1 in 5 chance per subsequent trade. Once a villager is willing, they will immediately seek another willing mate, if the village can sustain more villagers (based on the number of houses).

There is actually no predefined amount of time you wait for villagers to breed. It is random, but depending on how many villagers you have, you could see them breeding at a fairly quick rate (even as fast as one every 2-5 minutes).