not 100 % sure on how often but remove eggs or wrigglers and fatten the female up and they should start on a new batch.my issue is that they are slow grows.i have three pairs breeding for food and theres no way to raise them fast enough.but on the plus side u get a good amount of fry per batch.
Couple weeks if you remove fry. Couple months if you don't remove fry. Depends more on what you feed the female than anything. Feed shrimp to give her the extra minerals to create bigger batches of eggs.
I just commented about this in another thread.
After they start walking them around the tank, if I don't remove them, they will eat them and start again. Mine breed every 3-4 weeks. They take months to reach a size I can feed my fish and I find it's almost pointless.
Guppies and mollies produce less fry, but they reach a feeder size in half the time.