I asked some friends of ours the same thing, as they had two children, the youngest of which was four at the time. The response? "When she can climb out." She climbed over the edge (and fell to the floor, unscathed) for the first time when she was about 21 months old. She didn't attempt to climb out (to our knowledge, at least) again for about four more months. Then she seemed interested in climbing again, and let herself down pretty gently to the floor about three times in a week and a half. At that point, we went to a toddler bed (her crib converted to one).
I'm sure every child is different, so I don't know what you'll experience with regard to letting herself out of bed. Our little girl initially would get up when she heard the first one of us up and about in the morning, which was usually before she ought to get up, and it meant an earlier nap was in order on those days. But within about five or six weeks, when the novelty wore off, she resumed sleeping in and waiting for us to open her bedroom door before getting up, even if she had been awake and talking to one of her stuffed animals for twenty minutes. Or, from upstairs, over the baby monitor she would holler out
Mommy or Daddy, we would respond good morning, and yell back to her to get up and come see us downstairs, which she would do immediately. She's three and a half now, and that is still the routine. Without talking to her about it, she seems to have recognized on her own that when we come and get her in her room is when she should get out of bed, and no sooner (i.e. Saturday morning later sleep in for her parents is not a problem, provided the chit-chat on the baby monitor is not too loud or too entertaining :)
Good luck!