I'll give it a shot...
It depends on the temperature of the magnet over the period of time it lasts.
The cooler the better.....
If it stays cool and doesn't suffer chemical alteration or nuclear decay the magnetization can last billions of years....
The total magnetization I'm certain would fluctuate and weaken but on such a time scale that we might as well not worry about it practically.
The crust of Mars was magnetized about 4 billion years ago by it's transient core dynamo and we still strong crustal magnetization.