Warm-up time for amps and preamps

Not all SS amps are created equally.  As an example, my Gato AMP-150 has a warm up feature as they have determined that it takes a full 2 hours of operation to achieve optimum conditions.  With this warm up feature, it takes only takes 15 minutes. 

Small preamp tubes last longer if kept on as the turn on surge etc... shortens their life. Depends on build design, but I have owned tube preamps where the designer told me to keep the unit on unless traveling for best sound and tube life. The amp never got hot and the parts and tubes were cozy warm for long life and wonderful sonics. 

All my SS gear is on 24/7 unless aim way for a couple of days. The amp is in stand by unless i am using it.

Yes, frequent switching on and off in some ways can be more detrimental than leaving the amp permanently on. It depends on what components we are talking about as capacitors or transistors of certain amplifiers can last very long, well over 20 years before a degradation of sound quality is perceived. Naim recommends their solid-state amplifiers to be left permanently on, and the recommended service is every 10 to 12 years for the amplifiers and power supplies.

It may not be recommended to leave tube gear permanently switched on but some solid-state gear are designed to be permanently on without much adverse effects.