Tubes going bad???

Do tubes “go bad” slowly? If so, how do you detect when they’re “going bad”?

Some years ago I had a lesser tube system than I have today and was not so critical in my listening. As I recall, there came the point where I obviously needed to re-tube, my system sounded awful.

Moving forward, I’ve stepped up significantly in my equipment (Conrad Johnson’s Premier 16LS and Premier 12 mono’s)+ other tube gear. With consideration of the pursuit of system optimization (upgrades, tweaks, placement, etc.), how do I know if my tubes are performing optimally - sounding as good as it possibly can? If tubes do degrade slowly they may only be 97% today, 95% next month, 92% a month or so later at which time it becomes apparent it’s time to re-tube. If this is the case, I’ll have gone through a period of less than optimal tube performance that I did not catch on to until it became very apparent.

So, do tubes go bad instantly or is it a slower process that needs monitoring?
Unless a tube catastrophically goes bad, and you will know it when it happens, it's "normally" a somewhat slow process; the amp will begin to sound "sluggish" with a sense of loss of dynamics and general "oomph." Bass becomes less defined, with more boom than a pure note with texture. Depth of stage collapses as microdynamics and ambience are lost. Your words indicate that you care enough about this and my sense is that you will notice these things and hence will know when it's time. Power tubes go long before preamp tubes; rarely do I replace the preamp tubes due to loss of performance.
Simple and angst free method. Keep at least one set of spare tubes about. When you think your sound may be deteriorating insert the new tubes and see if there is a noticible difference (they should have about 24 hours to settle in). If you can't hear a meaningful difference pull them and go back to the old tubes.

FWIW I agree with Stevecham's comments.
I think the best way is what Newbee said, have a backup set.

I thought when I got my tube tester, that was all I needed. It does help with many things but I also found out that a tube "could" measure good and still sound not so good.

It's hard to notice the slow degradation and you are right, you may end up with many months of sub optimum performance.
Think of tubes as light bulbs, I never know when they go but I always have an extra bulb around. People make tube amps sound so difficult, your CJ are not difficult.
all great points above. I'll add when the gettering or the silver inside the top (or in some cases bottom as in some 6sn7) of the tube is almost all used up, ie turning black or dark it's time to think about new tubes.
Conrad Johnson actually has a recommended tube replacement schedule for all of their gear. I have a CT5 and they recommend replacement of its 6H30p tubes every 1500 hours or so for best performance. My ears confirm this.
Thanks all for the responses. Having a spare set for evaluating the set in use is a great suggestion. This also alleviates ever being caught shorthhanded.
First, you have two very good pieces of equipment. The Premier 12 can be upgraded and further improved so you can keep it forever. Call CJ and they can walk you though the whole process include tube replacement. Tubes last longer than you think. The sound will get slugish as stated, but also can sound distorted and/or make noise. Remember! not all tubes amps are designed the same so not all tube amps respond to tube rolling the same way.
One point is the big difference in the way most preamp tubes go, and the way 'some' amp tube go.
Preamp tubes almost always go out with a slow wimper. They just start getting noisy.. and you know it is time for a new set.
Amp tubes can go that way.. but then they can also go with an interesting or spectacular BANG !
Just sayin'

((I feel confident enough to leave my VAC Standard 4 tube (12Au/Ax7) preamp on 24/7. It is sealed so even in the super rare event a tube explodes or whatever, it will not burn my home down. On the other hand, I would NEVER leave a tube amp on, unattended, at all...))
How equipment will sound when tubes begin to show their age, depends on the way the equipment pushes the tube. If the tubes are run hard all the time, they will start to sound sluggish, muddy and lacking in dynamics much sooner. If they are run more gently, they can be quite weak and still sound pretty much the same as new. I have an amp that uses 6sn7s as drivers. These tubes can read extremely weak on my tube tester (Amplitrex) and still sound very good.

Also, if you tracked tube performance (e.g., transconductance) against time of use, you would find a broad plateau--over most of the tube's lifespan performance does not change much, but when they finally do start to decline, the decline is, comparatively speaking, much faster than the very slow decline during the plateau period.