[Dacious]

Other C30 users might like to pipe up, but they don't seem especially hard on tubes. It's always a wise policy to keep a spare set and one preamp tube (12AX7) too, as you might a spare fuse; as like a fuse they should be regarded like a consummable.

How long is a piece of string? Depends on quality of manufacture, individual tube construction, where the tube is, in what amp and what it is doing. Good quality preamp tubes for example might be virtually immortal. In some positions they wear out faster than others, like the 12AT7 reverb driver in BF/SF Fenders which wears more frequently than other preamps in those amps.

Power tubes are another matter. Think of tubes as like spark plugs in your car. Lots of flat out running will wear them out faster than a little leisurely use. If you play once or twice a week at moderate volume you might get up to dozens of years. OTOH if you play ten hours a day dimed with a metal pedal through the front (any gain increasers will work the whole amp harder) you might get hours-weeks.

You should always get matched power tubes, and always replace them as a set. So you would expect to replace them sooner than say a 6L6 amp. Running worn out tubes too long, like running worn out sparkplugs, is not wise from a longterm maintenance POV. I would ask for low-current draw tubes which are more likely not to cause a problem. Getting them from a reputable vendor lis encouraged - tell them what they are going in, often the vendor builds a knowlegde of what works best. Sovtek EL84Ms don't have the best sonic reputation but they are tough tubes. JJ Teslas are another with a good rep. EIs are reputably sonically good, but maybe not so tough.

Increases in hums, or popping and crackling especially if it occurs when volumes are at zero is potentially a tube issue - swap your spares in temporarily and check. Same with noise when voume goes up - try swapping your spare preamp in to each position. If noise goes away - walla. If not, might be another issue.

Generally, if the amp starts sounding muggy, fuzzy, you notice it seems to have lost clarity and volume is a sign the power tubes are on the way out as is silence if amp lights and tube heaters aren't out. Swap spares in - if problem solved, buy in new spares, 86 the fritzed tubes. If no difference, might be something else. Often a blown fuse might indicate a tube issue - swap in spares, good? Problem solved. Not? Tech time.

To maximise life, warm the heaters up for ten minutes on standby or zero volume before playing, and leave amp on standby or volume zero'd if taking a short break of less than 30 minutes. Tubes like car engines prefer less cooling/heating cycles.

Watch out swapping hot tubes - they burn! Oven mits are good to avoid burns. Hot tubes should be handled minimally and not jarred or banged into things.