2 or 3 years minimum searching before they find it probably with Air France they found wreckage a few days on the surface and that gave them the general search area. This is really worse than looking for a needle in a haystack.
The first discovery of the Air France wreckage was made after less than 24 hours from the crash. They recovered an aircraft seat, an orange buoy, and a barrel. Objects such as aircraft seats are sufficiently large, to be spotted from a surveillance aircraft.
They probably had a much better idea of where it was though. They seem to be still pretty uncertain about this so are searching a much larger and extended area.