Over the years with in-house experimenting and working with over 20,000 clients, we arrived to a count of 50 extractions as an average to perform a backflush routine with detergent on a backflush-capable espresso machine. More frequent backflushing without detergent can happen more often, but the frequency needs to be considered as a higher frequency can have a detrimental impact on the internal parts, especially a vibration pump, of an espresso machine.
Again, the count of 50 is an average. Similar to descaling frequency, this count depends upon other, but different driving factors:
Oiliness of the beans
The more oily the beans, the more rancid oils will accumulate behind and around the dispersion screen. Hence, you may need to backflush more frequently.
Finer grinds, especially powder resulting from an inconsistent grind, will accumulate behind the the dispersion screen. Hence, you may need to backflush more frequently.
Amount and type of detergent used
Some detergents clean better than others. Too much detergent can clog the grouphead. It can also strip out the lubrication on the cams in the e61 lever grouphead (other groups may not be affected). Hence, if you use a lesser or weaker detergent, you may need to backflush more frequently.
If you leave the puck to dry in the portafilter in the grouphead
If you leave the puck in the portafilter in the grouphead, the rancid oils will accumulate with faster and greater intensity. As a result, this will require a more frequent backflushing routine.
Technique (short vs long)
Typically, we recommend a long backflush routine for rotary vane pump machines. We recommend a short backflush routine for vibration pump(s) machines.