We try and keep the ewe with their lambs in the pens for 3 days, but during the rush part of the season it doesn't happen.  Some get kicked out after two days.   Everything that gets kicked out that particular day goes in a small pen, sometimes we have to use two pens if we have a lot.   Anyway, they stay in the small group for a day or two before the small group moves to a larger group.  After two weeks, we don't add any more to the large group and start over.  The key is to keep moving them to larger and larger groups, then everyone seems to happy.