What is the relationship between distance from the magnet and the strength of its magnetic field

Magnetic force obeys an inverse square law with distance. The equation for magnetic force is similar to Coulomb's Law (if you are familiar with it). But the key point is that the force is inversely proportional to the distance squared

force required to detach it is given by

charges are taken to be fundamental charge

now we have

so above is the minimum distance at which it will detach