Vacuum tubes or valves (UK) have a number of factors that contribute to their life span:
1) integrity of design
2) choice of materials
3) integrity of manufacturing
5) physical shock
6) electrical stress
7) number of hot to cold, or cold to hot cycles
Integrity of design factors should be considered, tubes are optimized for performance or optimized for longevity.
Materials factor as matter of cost vs performance.
Manufacturing practices factor in, as you might have the best design, and the best materials, but the assembly is not optimized. Examples can be found by comparing vintage NOS tubes like RCA, Genalex, Mullard vs. many overseas brands made today and this goes for design and materials as well.
Cooling. The internal temperature of many power tubes is close to 800 C which generates a considerable amount of heat on the exterior side. Consider early VOX amps of the 1960's with a cabinet/chassis design that failed to provide adequate cooling and hence started nasty fires on stage. Consider how vintage 1960's Fender Black Face amps had wide openings near the tubes to allow for air flow. I believe there are a number of later model amps such as the Mesa The Lone Star® that use a fan.
Physical shock. Pinging a tube too much may damage some of finer internal attached materials, some tubes are by nature very microphonic as well. Always test this before purchasing by a light tap with a chop stick to see if it can recover with out ringing. I once tested a set to RCA Meatball logo 12AX7s preamp tubes (circa 1955) that were so microphonic that one actually acted more like a ring modulator.
Electrical shock. Exceeding the spec sheet on a tube's B+ can kill or severely shorten its life. Consider one of the key differences between Leo Fender's "tweed" amps (1950s) vs. his "black face" amps (early 1960's), which was the B+ voltage.
The "tweed" amps had their B+ supply at about 270 to 350 vdc, while the "black face" amps were around 350 to 480 vdc. This was done to extend the head room so the "black face" amps tend to play clean all the way to diming them out. Leo's tradeoff was that to get the head room he had to put the power tubes on the upper margin of their spec. In the 1960s, utility companies in the US were typically setting the outlet at around 110 to 115 VAC, but today this is closer to 120 to 125 VAC, hence the power supply for the B+ is even higher. Cautious players of these vintage amps will put a variac between the outlet and the amp to keep the input voltage closer to a 'vintage' value to save the tubes as well as best match the optimal tone.
With all this said I have not answered your question yet. I just wanted to start with a look at the bigger picture. I personally love vacuum tube amps because of the rich, warm, and vibrant tone. This works particularly well with Blues as for the way tubes produce a more pleasing set of overtones than solid state amps when driven into saturation/distortion.
I understand that your Marshall amp does not have a standby switch. All the vintage Fender amps that I have restored already have a standby switch. In these designs, the standby simply shuts off the B+ while leaving the 6.3 vac heater voltages live, allowing all the tubes to stay warmed up. I generally allow 2 minutes before engaging the B+ this helps save the tubes. Remember tubes are highly inefficient with regard to power, they release 70 - 80% of the energy they consume as heat.
I recommend that once you turn on your amp, leave it on until the gig is over unless you have a 2 hour gap (this is an arbitrary value, in a 1 hour gap I might turn it off, I have no reliable data to support 1 hour or 2 hour). Be sure the part of the amp that has the tubes is adequately vented, if you have a small fan that is not going to interfere with the music this will help keep things cooler and the tubes like that part.
Other things you can do. Purchase the best tubes you can afford (most vendors of vintage tubes will have tested matched pairs and will take them back if these fail on delivery), make sure the power tubes are properly biased*. Don't store your amp in the trunk of your car other then to transport it. This will assure that the tubes do not suffer undue physical shock and extreme temperatures. Likewise, don't store the amp in a place that gets extremely hot or cold.
At the gig, measure the outlet voltage or mains to be sure it is an acceptable value, e.g. above 100 VAC, and below 125 VAC in the USA. Also make sure that the venue has grounded outlets and that your amp has a chassis ground with a 3 prong (ground, hot, and neutral) plug for safety reasons.
- biasing power tubes can kill you if don't know what you are doing. There are lethal voltages in a tube amp. This is true on many amps (all Fender vintage amps) even when the power has been removed. Take your amp to a qualified amp tech to have the tubes biased and checked once a year.