Well, I don't know that there's a scientifically proven "right" answer to this, other than when they pocket or break. I change tom heads yearly unless I pocket or break (which only happens if I try to go single ply...I should know by know I hit too hard for that shit). By year's end the coating's mostly gone and I usually have a few bead dents. I rarely change kick heads, though those do weaken after time and dull. Snare heads I tend to replace yearly, and are usually in the same condition as the tom heads. Bottom heads I tend to not do anything about, though like kick heads they eventually lose flex.