I once found a good schedule but I can't find it so I'll try to remember.

Once the female gets pregnant, it takes roughly 3 weeks for her to carry the pregnancy. Then she gives birth and nurses for about 4 weeks (I plan on pulling my feeders at 4 weeks and holdbacks at 5). Now the schedule I saw gave females a break between litters to regain weight and have a much needed rest. It gave them one week, so from start to finish its about 8 weeks. If you are introducing females to males every week, you "should" have a litter a week if you have at least 8-9 females. Of course, animals never read their own manuals so this might not go exactly according to plan.