How can you tell when you need to replace tubes?

I did not realize that tubes take 1000 hours to break in until I experienced it my self. I can say it has been that long, but the tubes sounded harsh and cold at first. Now the EL tubes sound very detailed with good layering and seperation, plus there is no harshness or distortion like at first. I though EL tubes were not very good until many hours were on them.

Thanx,