You can hold them within 24 hours. The mother is usually okay with this and will allow you to do so. You need to check and make sure they are being fed. If they are wrinkled and skinny and lethargic, you will need to do something. If they are being fed, they will be hopping around and squealing, even if they're eyes aren't open.
About three weeks
You should wait about three weeks. Wait until they are weaned. That shouldbe three wweeks.