I put my female rats into production at 16 weeks old and I leave them in production for 48 weeks.

So they produce from 16 weeks old until 64 weeks old.

I replace 1/3 of my breeding rats every 16 weeks.

This is the system that has worked out best for me.

People will tell you to put the females into production earlier, but I don't like waiting around for things to happen. When you put a mature male with four 16 week old females you usually have four litters within four weeks. If you put them together when they are 8-12 weeks old you may not get babies for months and months and months.