As often as you upstep it
Here is one datapoint with a deck stepped mast. My mast step being a crumbled pile of white dust may have been prevented had the previous owner unstepped my mast with any frequency (if ever). So, I can say with authority that it should be done sooner than every 37 years.
If someone else was repairing my cabin-roof and building a new mast step, I would expect the cost to hit in the neighborhood of 1000-2000 dollars. That is just a SWAG based on what it's taking me to get there. That money could have easily covered unstepping every 3-5 years for the life of the boat and a couple cases of beer.