My kids have done it from early on, dd1 was probably a few months old and was about 6months. It was harder for her as she hated to take a bottle. Haven't done it with Ds but it's hard to expect them to take all three.

I think it depends on the relationship they have with the grandparents as well as how you feel about it.

All three of mine were scn bubs and were used to being cared for by a tribe of people, it is something we have carried on with after they got home. It has meant dh and I could take a bit of time out together on the odd occasion and the girls get spoilt by nanna.