It depends. I don't keep real good track of how long a group has been together, but when they start looking really tattered around the edges and stop producing litters regularily it's off to the burmese retirement home.
I've constantly got young females growing up and the burms need to be fed anyway so I probably don't get as many litters out of them as I could.
As said before it really just depends. All of my first colony lasted for about 1 1/2 to 2 years of breeding them before I started noticing smaller litters or no litters at all.