To a negligible amount in my experience. Many electrical motors have an official shelf-life of around 10 years, due to other factors. If the magnet is treated correctly, it wouldn't loose much strength, maybe 1-2%.

Some magnets lose strength from concussions and higher temperatures. A very strong magnetic field can also damage the magnet. That said, motors with permanent magnets have quite good performance properties and are pretty energy efficient.

And if your magnet fails, you can always revitalize it with an external magnetic field.