Actually, tube amps absolutely do not mind being on for a few hours at a time. Now, if the heat removal system of the amp is bad in the first place that's a different story... but would you ever worry about playing your tube amp at full blast for a 2 hour set? Well, if you have an amp that you do need to worry about lasting for 2 hours, you need a new one! Any decent amplifier should be able to play for that long with no issues. What's ten minutes compared to hours of actual use?
Sure, put a tube amp on standby. Having high voltage somewhere where it's not really needed isn't really a good idea, so don't take it off standby unless you're using it. But you should not have issues if you do that. On some DIY tube amp forum somebody mentioned that they found a tube amp that had been doing PA duty in a school, that had been switched on for 27 years (the person maintaining it died and nobody knew it was there). It still worked. The life of tube's filaments, the things that are in use if your amp is turned on but set to standby, will last far longer than the tube's usable lifespan in most cases. The filament in a power tube should last several thousand hours. Are you really going to play one set of tubes for several thousand hours? No, they'll sound bad, give weak output, and need replacing far before then.