How long will a permanet magnet stay magnetized?

Question

And if there a set time how can that time be calculated?

Answer 1

Almost indefinitely. SInce room temperature is high enough that atoms are still moving around, the magnetic domains will suffer as the motion rearranged the atoms. Often magnets come with a "keeper" which is usually steel. The keeper helps by concentrating the magnetic field. Even if a magnet looses if magnetic strength, it can be remagnetized by exposing it to a high magnetic field. Magnet materials also have a Curie temperature, a temperature where the vibration of the atoms will "erase" the magnetism (by randomizing the magnetic domains). Again, it can be remagnetized once it cools.

Answer 2

Permanently! Unless you hammer it or heat it nearly red hot.

Answer 3

indefinitely.