I presume demagnetizing a magnet would require about as strong a magnetic field as does magnetizing a magnet, which is clearly a lot, but it depends on the magnet. If it's "ordinary" iron or steel that's magnetized, like a screwdriver magnetized by touching a magnet to it and that can then pick up steel screws, a "moderate" AC magnetic field will demagnetize it. I've read that ALNICO magnets are relatively "weak" and easily demagnetizable, but not so for most magnets.
