It's different for everyone. I'd planned to go back to work when DD was 7 months and, for me, it turned out to be too soon. I extended my leave for 2 weeks because my mum died 2 weeks before I was due to go back. I worked for one week then had 6 weeks off with glandular fever. This time I'm taking 12 months, but DH and I will reassess things towards the end of next year because I'd like to stay at home if we can afford it.