In the real world most commerical airline raise gear when they have established a postive rate of climb. As for landing any where from 50 nm out to say 75 nm out.
Just make sure to check your air speed and you should have at least 10 degrees of flaps down first. Altitude should be somehere between 1200 and 3000 feet. But you can raise or lower the landing whenever you want.
But unless you do not want landing gear do not lower gear above airspeed of 250 knots as if you do you probably can kiss them bye bye.
If you have realism turned on the sim will not allow you lower your gear if are going to fast. If not you will hear a warning sound telling you that you are about to rip the gear off the plane.
