Small-signal tubes are usually rated for about a 10,000 hour lifespan, give or take. They will often last a lot longer, but they start to get noisier and sonic quality does start to degrade. So if you leave your preamp on 24/7, 10,000 hours is reached in a little over a year. If you are willing to retube your preamp every year, then no problem. Otherwise, turn it off when not playing, and you'll probably get 10 years or more of tube life. Power amps are even worse, with output tube life of a few thousand hours and much higher replacement costs.