47 years may be a bit long, but every three or four years should be more than enough. The major reason to take the drums and bearings off is to check the brakes. Bearings should be good for 50,000 miles at least if properly packed, but while you have things apart, checking and repacking the bearings won't hurt if properly done. Be sure to use some kind of pressure packer to pack the bearings. Its too bad that you can't remove the drums without disturbing the bearings, then you could leave the bearings alone for many years.