The typical recommendation is every five years. I can't say I ever did. But I ended up replacing a lot converting over to small and natural cell. The easiest time to pull comb is in the spring when most of it is still empty. If you really want some comb out that is not, move it to the outside edge of the brood nest until it's the outside frame and when it gets backfilled with honey and there is a flow going on, pull it.