Hi there, and welcome. I wish I had discovered mumsnet before, or even during my pregnancy!
I know this is not terribly helpful, but I don't think there is a categoric answer to your question: it's whenever it is right for you.
I think I was really worried when I was pregnant that my life would never be my own again, but dd is now 6 months old, and I don't begrudge a minute that my life revolves around her. I just count myself eternally grateful that I have no financial pressure to return to work. But then, I had already given up a career to do my own thing, so that was not really an issue for me.
With regards to getting fat, I actually found, to my surprise, that when I was pregnant, I was happier with my body image than I have ever been: I was pregnant and proud of it. This must have showed, because I had more compliments on my appearance when I was pregant then before or since. What I did find really weird was the changes taking place in my body, especially as for the first time in years my body was doing things that felt, well, alien, and because it was my first pregnancy, I didn't know what was 'normal'. I found the book 'What to expect when you are expecting' really useful, and in this forum you have the benefit of anonymity to ask all those questions that you are too embarassed to ask your midwife, which is why I wish I had found it earlier!
Dh was absolutely terrified of becoming a father, but now he is completely enchanted with his little angel. He didn't make an issue of the fact that for a couple of months after dd arrived, all I wanted to do in bed was sleep, but I guess that might be more of an issue for some men. It is a bit of a transition going from being a couple to being a family, but neither of us have any regrets about becoming parents.
However, the days of getting ready to go out and being out of the house within five minutes are long gone - it took a while to get used to allowing at least 15 minutes to dd's kit and dd into the car to go anywhere!
Good luck, whatever you decide.