A local dealer leaves their amps powered on 24/6. they shut down on sat nights. on monday AM opening the store the sound is noticeably off compsred to days after they were left on. the main rep says it takes two days for some of them to sound their best!
My incidences tell me they sound best after 45 minutes or so
I am not leaving my amps on 24 /7. I have a 40 year old pioneer receiver that still sounds great. i doubt it would be in the same condition being left on for 40 years.