The last time I replaced a picture tube in someone else's solid state color set the TV was running 24/7, and was pushing 8 years of age at the time.
Maybe someone else can come up with an answer for a tube specifically from the era of your set, but I've never seen specific use life in hours advertised. Most CRT manufacturers seemed to boast "fewer callbacks" with their premium tubes. Back when your set came out there probably weren't that many hours of broadcast per day for most stations, thus nothing to watch, thus TV on only perhaps a few hours a day, max. I think the real answer to the question is God only knows; there are a lot of variables here.
If the tube you have in there now starts showing signs of losing chutzpah you have chances of stretching its use life by rejuvenation and the use of brightener transformers. I'd stacked as many as 3 of them to boost up the filaments of a 21FJP22 in an old Zenith color set I had. You could practically read by the light of the filaments, but I got another year out of that tube before I finally was able to find a replacement, and some time to do the swap and do the gray scale/purity/convergence adjustments on a Saturday morning. Those roundie tubes are big and heavy bahstids, BTW....
