When to replace tubes in modded Denon 3910?

I have an Exemplar-modded Denon 3910 that I bought over a year ago. I'm interested in other people's experience regarding how long the tubes last. And even more importantly, I guess, how does one determine that the tubes need replacement? Is it a no-brainer, i.e., they start getting obviously noisy? Or is it sort of a subtle thing, e.g., the system doesn't seem to be sounding quite as good as it used to? Thanks!
I own one, too. I've also owned three other tube CDPs, and I've never had to replace a tube in any of them. However, the two times I've had to replace a tube in my amplifier and preamplifier, it was marked by either a crackling/spitting sound, or by hum isolated in one channel.

Honestly, I think tubes in a CDP can last several years or more.
If you have doubts about your tubes: Find a musical equipment repair shop in the local phonebook. Most will repair tubed gear and have a tube tester on hand. Tubes of most families vary widely as far as life expectancy. For instance: While a 6DJ8(most brands)is expected to last 2000hrs, the E188cc or CCa(same family/higher quality/interchangable) are 10,000hr tubes(and- much more musical, if 60's Siemens or Telefunkens, ie). Noisy/spitty tubes have problems other than age generally. Yes- there will be changes in sonics, and levels with age.