When Thor Audio was around I talked with Paul Marks, its founder. His view was leave it on 24/7 and just change the tubes each year. So that would be about 9000 hours of use. Make a guess at how much you have used them and that's a very ballpark estimate of whether to get new tubes. Sound is another good guide.
Tube Life
I’m using a phono pre-amp with a pair of telefunken 12AX7 and one 12AT7. I purchased these tubes several years ago and they were rated at about 50% life. Are tube noise and tube rush the only reasons to change out tubes? Everything is silent and yet I can’t help but think that several years of use should indicate some sort of wear. Any experience or suggestions will be appreciated.