Yes, around 48hrs penned up with their lambs is usually enough, just make sure their bonded. I always turn out into a larger communal pen then, which is always chaos for the first 10 mins as lambs have no idea that they need to stick with their mums, so they get pushed (or shoved) around a bit, but they get the idea pretty quickly.
So when to turn out. There are lots of variables, how old are the lambs, how much grass do you have, do they have shelter in their field and what is the forcast for the next few days?
Ideally I like my lambs to be at least a week old, I've had problems with foxes here taking young lambs. If the weather isn't good with you could you turn them out during the day and in at night? When I've been able to do that its been great, but sometimes you get a ewe that decided she doesn't want to come in. You need to avoid any prolonged wet spells, or wet and windy, cold is fine, although I'd be concerned about frosts?