The landing gear should be down in order to qualify as a stabilized approach. Other than that, what are the guidelines on when to deploy landing gear on approach?
As late as possible to still meet your company's specific stabilized approach guidelines (mine were 500 ft VMC, 1000 ft IMC). If you need the drag, then use them earlier.
In the EMB-145 in still wind or a headwind you could cross the FAF at 250 kts and clean and be on speed and configured by 1000' if you dropped the gear at the FAF (and opened the speed brakes and the first notch of flaps all at once). Normally though, I'd drop the gear around 1000'.
- VMC = visual meteorological conditions
- IMC = instrument meteorological conditions
- FAF = final approach fix
- EMB-145 is the type designator for the Embraer 145 regional jet
In light general aviation aircraft, you lower the gear when abeam the touchdown point on downwind, then double-check that the gear is down on final (and possibly on base, as well).
It should normally be dropped about 500ft - 1000ft and below the aircraft's maximum speed for dropping the landing gear so you don't damage it. If you need more drag to slow down quicker you can drop it a bit higher up.