How often do you clip a parakeet's nails?

The best way to answer this would have to do with a visual inspection of the nail and what the nail touches when the bird is perching on a limb, perching device of some sort. Ideally, if the bird is perching on something round, and if the nail touches the back part of the foot while perching, then the nails are too long. Typically, you can see if the nail is clear, where the vein runs and you of course would never want to cut into the vein that supplies the nail with nourishment, oxygen, through its blood flow. If still not sure, don't risk it. Take the bird to your local vet and observe and watch them do one foot and then you do the other foot with their supervision. This is the all time safe guarded way of providing this service to your bird.

In the wild, the bird naturally keep their nails and beak filed down by the things it chews and claws upon while resting/perching. You may want to incorporate some of these items into the cage to assist in the natural way of filing them down.