As MR said, it is a really personal decision.

I left my DD with my sil when she was about 3 months old cause I was so sick and DH was facing redundancy at work and couldn't risk taking the time off. A bit different I know..

But I had to go back to work when DD was about 4 and 1/2 months, and the only way my Mum could watch her one week was to have her at her place for 2 nights. (she lives 2 hrs away).. As hard it was I agreed.. But now, she is 2 and I can't bear the thought of her staying anywhere without me (might be cause I am now a SAHM).