Depends on the tube manufacturer and your equipment. My Jadis amps seem to eat up every 6550 (ARC Phillips, which lasted 5 years in my ARC M300s, and Svetlanas) I've put in them within 12-15 months, but I've been using NOS GEC Gold Lions for the last 3-1/2 years with no problems (the Jadis have a fuse for each tube that protects the amp if the tube goes bad), and I know of a JA200 owner who's had his Gold Lions in his amps for about 7 years. Don't know if the Mac is as hard on tubes as the Jadis, but I suspect not--I recently bought some Gold Lions used from a McIntosh owner who'd used them for a year, and they tested as strong as unused tubes, and I also know of another Mac owner who kept his amps on all the time with 6550s and no problems for years! More recently manufactured KT88s are reputed not to be as long-lived as the old stuff, but I haven't tried them. As far as tubes wearing out, I usually go by the fuses in my amps, but I've got a good tube tester coming from Andy Bouwman soon and you might want to consider getting one as well. Otherwise, I've found worn out tubes seem to lose a bit of their high frequency extension, dynamics and life when they started to get worn out (and before they started glowing cherry-red); I'm sure others will chime in here with their experience. The change is usually so gradual, though, that you often don't really notice it till you put in new tubes, as you become accustomed to the sound.
