How long do NOS 6L6gc power tubes last in a Mcintosh MC240 amp?

The 240 is very easy on tubes as long as you put good quality in AND your caps are in good shape. Like 2-4 k hours, you can see a nice rebuilt 240 on my vintage system page.

a fantastic amp and very sweet w Telefunken in the small signal. I am currently running some output tubes I got from Andy at Vintage Tube Service in Michigan, he has the chops and the test equipment to know ;-)

have fun, enjoy the music

But the skeptics will ask ( fair ) how do you know ? 1961 Amp store demo came to us in 1965 and went to McIntosh clinics regularly, I remember over many many years a few tubes. I inherited it in 2001 and it finally needed another set. I put the Vintage Tube services set in 3 years ago :-) she beats her spec by 12 wpc

Also, if you don’t listen very loud, try light loading the output transformers but dropping down another setting on the impedence- listen also as that is the key - if it sounds better w no clipping keep it that way.

tomic601 is correct. the MC240 is pretty easy on tubes. not sure what speakers you are using but try both 8 and 4 ohm taps. usually one will sound a bit better. Also try some KT66s in the 240.
not sure if your 240 is original or not, but greatest improvement you can make is to add a film cap to the 100uf electrolytic cap in the feedback circuit.
Takes longer to remove the bottom cover than it does to solder in the cap, one for each channel.

Thanks @johnss and @tomic601, I have Klipsch heritage Belle speakers, and just ordered replacement tubes from Brent Jessee.