The fish need to be at least 8-10 months of age, have a body size of at least a half dollar coin, and show a desire to remain together. If 2 fish do pair, the male of the 2 will start attacking the other fish in the tank to keep them away from "his gal". If you have a longer tank, you will see 2 fish on one side and all the rest of the fish on the other side of the tank. If the other fish don;t have a place to get away from the pair, there will be some damage to them as breeding males can be very territorial. It's always best to separate any potential pair or actual pair out of a community tank situation.
As for time frame, there is no set time frame. It's all according to your tank situation.
Hope this helps.