I currently have a rack with 35 "units" so I guess I raise a fair number of mice. Just so you have an idea how I do things, I put the newest young colonies in the top rack, and as I retire older groups from the bottom rack, all the rest of the mice move down a rung. I usually expect that by the time I move a new colony to the second row down, they're showing signs of pregnancy. If they make it to the third row down, then something is wrong and I check genders again to make sure I don't have two males or no males. Depending on how many mice I have willing and able to form new breeding groups (and how many adult snakes I have that are hungry) I will either give them a little longer, or feed them off. Unless the mice have some trait I really like, I just feed the entire group off. Mice usually spend as much as a month on each rung of the mouse rack, so the mice that are in the bottom rung can be as old as 7 or 8 months old. Those who produce well and seem to be continuing to produce don't get fed off until production drops off.
I usually start my groups at 1.6, and frequently attrition takes one or two females and by the time they get to the bottom of the rack I usually have groups that have shrunk in size to between 1.2 and 1.4.
How I start new groups? Well when the mice are allowed to live beyond pinky stage, I tend to keep an eye out for when they are able to fend for themselves... so they are usually removed between 21 and 28 days after birth and put into what I call my "weaner bin". All weanlings go there. Ocassionally when I have a group of mice that has experienced a larger attrition than average (more females dying, escaping, whatever) I sometimes leave one or two female hoppers in with the original breeding group. In general however, I try to get them out of the parent colony as soon as they reach 21 (days that is). When it's time to establish new groups I tend to pull the biggest and healthiest weaners from the weanling bin, one male and six females. Sometimes they thrive, sometimes they don't. Those that aren't thriving get fed off, those that are get a nice little life in the mouse racks.
As we're getting into the summer months here in Arizona, I expect to have a higher attrition rate. I purposely put the weaners in the top bins, and the oldest mice in the bottom bins, as it's warmer in the top bins of my mouse house, than in the lower ones. Even with the AC running, it's too darn hot to raise mice efficiently in the summer. That's why I stepped up my mouse production in February and started freezing large numbers of the critters... IN case I don't get much production, I'll still ahve plenty of mousey meals for my snakes.
