We tell all our customers to change out the valves every year and sell them to some one else or give them away. We have found two items that affect valve longevity (other than a defect in craftmans ship). One; too loose a clearance in the valve guide. if the guide is too loose, then the valve will seat differently each time and flex it at the stem to head position. Another problem is to little seat and or pressure . As the valve slams on the seat, if there is too little pressure to keep it there it will bounce for any number of times and we all know the alterntives. We work with our cam grinders to try to set the valve down on the seat just a little bit more gently. Makes things last a lot longer.
All this is my opinion only.
racear