Baby birds fly at different rates depending on species, but they typically take at least two weeks after hatching. Some species of baby birds leave the nest and wallow on the ground for a week or more, still dependent upon their parents. Some species fly immediately after leaving the nest.
One way to discern if a baby bird has left the nest too early is by looking at its feathers. Full-feathered fledglings on the ground can thermoregulate and survive with their parents' assistance, even if it takes a full week once they have left the nest. The less-feathered the bird, the less likely he is to survive outside the nest.