How long till Apistogramma mature?

You should get 4 FM if you have the room, ideal is 1m to 3f
One thing, apistos will guard their territory and you will start seeing aggression as soon as you introduce a female.
The males like to keep a harem. LOL
 
So what you are saying is that the male will breed with several females at one time?

My tank is a 20 gallon and it houses only the 2 males. How many females should I put in?
 
In 20G, I would just keep 1 pair. Once they start mating, not enough room for others.

As far as wild Apisto or other dwarf cichlids, majority of all the wild specimen I encountered was somewhat drab to selectively bred color morphs.
You could swap one of male for female and females are usually lot smaller than male.
 
4 females & 2 males in a 20 long is too many I think. Females can be very territorial. I'd try 2 females with the 2 males & lots of visual barriers, plants, rocks, wood. I had 2 pair in a 55g, 1 pair took the middle & the other an end but only 1 fry survived while I was on vac.
 
right now, it is between cichlids.net and mainlycichlids.com

I got very similar quotes from both places so I'm just deciding between which one to pick
 
AquariaCentral.com