When to replace tubes on a CD player?

I have a CD player (Ah! Tjoeb 4000) that uses two tubes in its configuration. Can anyone let me know when the tubes "are expired" for lack of a better word. I'm using Mullards at the moment. Cheers...Dave.
When they become noisy or the high's/upper mid range becomes dullish, that is they lose the luster inherrent in tubes.
If they were new when you put them in, they should last 5000 to 10,000 hours! That's a lot of years. Newbee's response is what to look for in the future.
Years? Not necessarily.....if the player is left on all the time, which is not unreasonable, if one wants to have the player warmed-up and ready to go all the time. 10,000 hours is about 14 months.