depends on what breed the ewe is, most hair sheep are out of season breeders and will breed back as soon as 4-6 weeks after birthing their lambs. Most wool sheep are seasonal and wont come into heat until fall, but I know there exeptions to that rule.
I keep my rams with the ewes, never had an issue with them bothering the lambs, only ones that worry me sometimes are the yearling boys trying to play with the 2 week old babies, but if that gets too rowdy the ewe gives them heck.