Usually at least an hour in a hot oven - 180 to 200C or 350-400F.
An hour and a half is ideal.

You can cook one in the microwave for about 7 minutes on high, but be sure to slice or poke holes into the clean potato. Then wrap the potato in aluminum foil.

You can cook a potato in the microwave, but it will be steamed, not baked. (Wrapping it in foil causes it to steam, too.) If you like crispy skin and a fluffy potato, it's worth taking the time to bake it uncovered (or you can rub the skin with oil) in a conventional oven at ~400F for at least an hour.

I stab them with a fork and put them directly on the grate/shelf in a 400 - 425 degree oven. The skin gets really crispy that way. depending on the size, check them at about 45 minutes if they are small to medium, an hour if they are larger. Bake them till done to your liking.