It will most likely take about a month before you will notice a difference in muscle tone after pregnancy, after various forms of abdominal exercise is followed. The time should be at least 20 minutes a day 3 - 5 times a week.
as often as you can. don't exercise the same major muscles everyday though. they need rest. example, monday you work back and biceps. then Tuesday do legs.
There are several exercises that people can do to imporve muscle tone. The first thing is to check with your dr and get a physical and than discuss a program with him or her.