Well 12 weeks is the legal requirement, some jobs get more, but generally 3 months is maternity leave for the most part. I'm lucky to get 6 weeks at my normal pay, because most don't get a paid maternity leave. I went back to work when ds1 was 4 weeks old and when ds2 was 3 months old, who knows if I'll go back after this one though. I told my boss at work, and she was fine and understanding and right now we're leaving it open.
the sad part is, is that most people really can't afford to be off for a long time, because surviving on one salary is impossible.