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?