Life span of tubes?

Has anybody experiences with KT88's' life span? I run an McIntosh MC 2000 about 6 hours a day average. When do I have to worry about the tubes?
What happens first when a tube indicates it won't go on much longer?
Chinese KT88s are not so good and last about 1 year(1500hrs at most) Teslas(Chekhoslovakian tubes) will last arround 2.5 years or 4000 hrs. If you run your MAC 6 hrs per day, do not turn it off at all -- keep it 24/7. Try not to allow operation with deffective tube(s) on your amplifier and check your tubes with tube tester every 6-7 months (1000hrs) from the time you get it.
It is a good practice to detect a defective tube earlier and replace it rather than to have every tube replaced at the same time.
Some tube amps might have electronic circuit brakers if the deffective tube is detected. Check with manufacturer or manuals on your amplifier.

For comparison if one of 4 cylinders of car engine is deffective, than most-likely your car will have the rest of three defected very soon unless you'll do the engine work on one cylinder.

Once the new tube is biased, it will have a very negligable difference in characteristics from tubes that are already played for some time.
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.
Life span depends upon amp design/application. Originally tubes were designed with a 10000 hour life span. if the tubes are used 'nicely' then they should last. I know that the old quicksilver 8417 monoblocks would eat tubes ~ a set every 10~18 months, but a Music reference RM-9 can keep the same tubes in place for 10 years. THis I also know to be true as I just replaced the tubes on my RM-9, and have had the amp since 1990.
The mac should be on the longer tube life end of the scale.
Happy listening
how much does tubes costs when replacing ?
i can think that there are lower and higher cost tubes much in general?

Depends on the brand (I'm assuming you mean KT88s here). The Svetlanas probably are in the $30-40 range, I think, from what I've seen on the 'Gon classifieds. Not sure about the Teslas and Chinese tubes, but probably similar, maybe a little less for the Golden Dragons. NOS Gold Lions/Gold Monarchs from Europe (not the Chinese ones from late in their history) are out of sight ($250-350 each, depending on their vintage and condition); if you can get used ones that test good, they are less expensive and can be a good buy.
Rcprince is right of course. Haven't seen any NOS Gold Lions or Monarchs for quite a while. Occasionly you can find one or two used at Billington's in the UK and they cost lots of Sterling. To get a feel for prices of other tubes, I suggest you check Ebay regularly to get a feel how the market is.
Detlof, that's because I'm hoarding over 50 of them! Told you I had a lifetime supply......
Lucky you Rcprince and a long, long life to you and your tubes of course ! (-: