The Cornell Lab of Ornithology states that songbirds typically incubate their eggs for two weeks. Once the baby birds hatch, they spend another two weeks in the nest before they are ready to leave, although different species often have different incubation and fledgling periods.
Fledgling pigeons remain in the nest until they are almost fully grown. By the time the young birds leave the nest, they are often bigger than their parents. This makes it almost impossible to distinguish adolescent pigeons from their adult counterparts. It's also why there is an urban myth that states baby pigeons do not exist.