I started my own business just before I got pregnant and I remember telling my business partner I would be back at work 3 weeks after the baby was born. Looking back at that time, I realise I was a little delusional. Even though I ran my business from home, there was no way I was ready to go back to work - I was sleep deprived and trying to juggle too many balls. So I would say, unless you have a full time nanny, give yourself a minimum of 6 months off, and if you can 12 months. For me it's now 12 years on and I still can't fit into my pre-baby office suits.
The company I work for has a generous maternity leave package so I was able to take a full 12 months off before returning to work. It would have been possible to go back earlier but I think younger than 6 months would be tough on the child (and mummy too!)
For some people it isn't a question of choice; financially they might have to go back to work sooner than they wished. I think most mums would love the time off with their kids but it's not always possible. I took a year off with my first son and this was just an amazing time and I loved every minute of it and count myself lucky that I was able to do this.
How long to wait is a very individual choice. Personally I have stayed home with my kids for four and I half years so far, but it would have been different if I'd had a job to go back to and been on maternity leave. I think I would have been pretty sad to leave my little ones before they were at least 18 months old, since that's about the age that when you leave they can comfort themselves by imagining you coming back, and it made it easier to continue breastfeeding. However, I realise staying home that long isn't an option that's open to everyone. If you have to go back to work after a few weeks or months, or you just want to that's up to you.
I'm male, so I wasn't allowed to take time off to start a family, unless I resigned. My wife had a choice though, thank goodness. It is also good to see the law has now changed in that respect, even if practices and discrimination against men hasn't.