The rate at which tubes wear out and how they sound as they do varies with the circuit they're installed in. The general tips above (and any that follow) are useful but if the OP wants the most applicable guidance he should post the preamp he's using these tubes in.
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.