Really curious to hear how people are chemical backflushing their machines and how often. We do 3 bursts of 30, 20, and 10 seconds at every close. We recently hired a barista from another shop who brought up a different way of backflushing as to avoid too much pressure on the boiler. He’s more in favor of shorter bursts, staying closer to the 10 second mark.
Is pressure on the boiler a legitimate concern? If so, is 30 seconds long enough to cause concern?
I’m used to several (5-6) rounds of 10 seconds with chemical, then rinsing 3 or 4 times without chemical, at every close, personally. This is across 3 shops, 2 with volumetric machines that signal when to stop pressurizing.
As far as how important it is to do exactly that, idk, but the importance of rigidly sticking to that exact method has never been stressed to me. I would suggest looking up and asking your machine’s company tech or someone similar, how the process works, kinda the physics behind it; to familiarize yourself with it all better. If it’s a big deal to you 
You’ve got me curious about it now…
Would normally just follow the cafetto / cleaning instructions on the cleaning product unless your machine has specific instructions to do it a particular way. Some machines have more automated cleaning cycles to use which help you out. The machine itself should be capable of handling the back pressure as it does this to release back pressure after each normal coffee shot. Granted when using the blind filter I expect it is close to full pressure but again, engineered to be cleaned and handle high pressures Definitely worth getting machine manual / advice from local tech / manufacturer. Guessing most machines should only need to be activated between 4-10 seconds (max) before enough pressure has built up. Not inclined to go beyond 5 unless the machine manual directs that way. Hope that helps.
I attended a talk with one of the US La Marzocco engineers and a well known machine repair technician. They felt you only needed to turn the pump on until the it reached full pressure inside the portafilter. 3 or 4 seconds. This is because the important part of backflushing is turning the pump off, which shoots the pressurized cleaning solution through the machine. They pushed people to use more cycles at a much shorter time because cycling on and off is what actually does the cleaning. They also recommended using a full Pallo brush scoop of cafiza in each portafilter, granted this was put on by cafiza so possible desire for them to have people use more product.
TLDR : 10 cycles at 3 seconds each