Harsh on tubes?

I came across something like "xxx tube preamp is harsh on tubes ..."
What does it mean by that?
Should I avoid such tube preamps if I am new to the tube world?
Which well known tube preamps are particularly harsh on tubes?
the circuit drives the tubes at their operating max shortening their useful life.
I think the word used here should be hard on tubes, not harsh on tubes. Yes, some preamps put a tough load on their tubes, which means that these tubes will have a shorter lifespan than normal.
Should you avoid them? That's up to you. Some of the finest sounding tube preamps for the money are hard on tubes. If you want piece of mind over and above sonics, then perhaps it is best that you avoid such preamps though.

Some designs run the tubes hard which can shorten their life.
If tubes are run hard (high current through the tube and/or high plate voltage) they tend to have a short life. Run them conservatively and some tubes have an incredibly long life. There are people who report that their table radios have been on for most of the day for 50 years with the original tubes.

Small signal tubes used in preamps typically last much longer than output tubes in amplifiers. But, some designs do run them hard so the tubes may last only a year in daily use. The good news is that when they go, you just have to replace the tube; there is far less chance of small signal tube failure causing damage to the component than is the case with power output tubes in an amplifier.

Whether you can live with an preamp that runs tubes hard depends on your own willingness to provide maintenance, whether you are uncomfortable with the fact that the tube and sound may be deteriorating quite quickly and you have to monitor the situation, whether the cost of retubing the preamp is reasonable to you, and whether the sound of the particular model warrants the extra cost/trouble.
Most tube preamps do not pass a lot of current through tubes, and thus the circuit, per se, is not hard on tubes. The issue is that most tube preamps have solid-state rectification (power supplies), and when the preamp is powered up, the tubes are slammed by the sudden application of voltage. This beats up the tubes quite quickly, some preamps quicker than others (the Audible Illusions is a well-known example). With such preamps, the way to avoid this is to leave the preamp powered up 24/7. Some tube pre's offer a soft-start feature which applies voltage slowly (e.g., CAT's preamps), but the tubes are still subjected to deleterious thermal cycles if the preamp is not left on 24/7, as they expand and contract as they heat up and cool down in response to being turned on and off, which is very damaging to tube or transistor life over time.

The question is more complicated for tube preamps with tube power supplies, as tubes in the power supply can pass a lot of current and thus face the same wear issues that output (power) tubes do, meaning that you may face premature tube failure if you leave the unit powered up 24/7. In addition, as tube power supplies tend to apply voltage gradually, not unlike a soft-start feature, turning the unit on and off does not beat up the tubes like tube pre's with solid-state power supplies. Thus, a reasonable approach with such pre's is to turn them on and off.

Tube preamps with tube rectification tend to be rare these days and are almost all quite expensive. The issue is really for the 95%+ of tube preamps that have solid-state rectification. To restate the point about what turn-on can do to small-signal tubes (the type of tubes used in tube preamps), the Wikipedia entry for "vacuum tube" has this to say about operation of the Colossus computers used in World War II to decipher enemy radio transmissions, which each used thousands of small-signal tubes:

"The Colossus computer's designer, Dr Tommy Flowers, had a theory that most of the unreliability was caused during power down and (mainly) power up. Once Colossus was built and installed, it was switched on and left switched on running from dual redundant diesel generators (the wartime mains supply being considered too unreliable). The only time it was switched off was for conversion to the Colossus Mk2 and the addition of another 500 or so tubes. Another 9 Colossus Mk2s were built, and all 10 machines ran with a surprising degree of reliability. The 10 Colossi consumed 15 kilowatts of power each, 24 hours a day, 365 days a year—nearly all of it for the tube heaters."

The Wikipedia entry for the Colossus emphasizes this point:

"Colossus used state-of-the-art vacuum tubes (thermionic valves), thyratrons and photomultipliers to optically read a paper tape and then applied a programmable logical function to every character, counting how often this function returned "true". Although machines with many valves [tubes] were known to have high failure rates, it was recognised that valve failures occurred most frequently with the current surge at power on, so the Colossus machines, once turned on, were never powered down unless they malfunctioned."
Many thanks.
But I am confused:
> the tubes are still subjected to deleterious thermal cycles if the preamp is not left on 24/7
> you may face premature tube failure if you leave the unit powered up 24/7

So, what would be a best practice to save the tube's life time? I listened to music 1~3 hours a day.

I have an int tube amp and will have a tube preamp that will work with a ss power amp. I would not mind replacing to new tubes every 4~5 years, but not 1~2 years.
Most of the tube people I've talked to say that turning off equipment prolongs the life of both tubes and the other components. It is true that, without tube rectification, the tubes are subjected to a pretty quick turn on. But, the thermal stress is NOT as great as that experienced by a light bulb, and so it is not as big a factor in tube life. With tube rectification, the rectifier fairly slowly starts to deliver current so there is even less stress to the other tubes making startup stress far less of an issue. In any case, you will be balancing two different factors that contribute to shortened life of the tube -- turn on stress and the inevitable wearing down of the filament/cathode that comes from use. Leave the amp on all the time and one increases that form of aging while decreasing the stress failure form of aging.

Also, heat is the killer of most of the other components of gear. Hot equipment shortens the life of capacitors and resistors and other components (particularly electrolytic capacitors).

If you are not sure which way to go, the manufacturer is probably the best source of information.