I do it twice a year. But I have a machine. If you play a lot of golf on a firm course, and hit a lot of balls off mats or firm turf, twice a year is right. But if you don't hit off mats you should do it at the beginning of every golf season.

Forged Irons, maybe every year or so. Stainless Steel cast Irons, I doubt change at all.

Frank P gave you the correct answer as far as I am concern.
You should also take in consideration the number of game played during a season.

Once a year with forged irons should be ok if you do not a problem with a specific iron.