According to the Department of Defense, a preamp tube meaning a small signal tube if played 24/7/365 will last about 22 years. At 8 hours a day, over 60 years.
It has been pointed out that temperature plays a roll here. It is not the tube that is in danger, but the other components. Heat kills capacitors (especially if your unit has 85c devices not 105c) film exception, and can weaken resistors even metal film ( wirewound the exception). The reason a 'designer' has suggested you leave something on is for thermal stability....everywhere has the same temperature, no cold/hot spots inside the device. Hot, cold, hot, cold also weakens or cracks solder joints.
All this applies to preamps. It still is a personal choice and how big your wallet is.
Power amps....no, no, no...period. Even 15 minutes is enough to get the temperature of the plates up to operational levels. Leaving a tube power amp on when you are not listening to music, is money in the bank for tube sellers. DOD rates power tubes at 3,000 to 5,000 depending how close to class A they are driven/biased from AB, and in class A the hours drop fast. This applies even in stand by.
Last, if you have a 'stand by' position for the amp, use it for the warm up as it keeps the B+ off the plates until temp is reached.
