Unfortunately, there are no basic parameters you can apply to cleaning intervals as it is a factor of machine, volume and operator technique dependent. A very simple test is to run some water through the group into a cup without the portafilter in the group. Smell the water. There will be 2 different aromas present - one from your boiler/thermoblock that resembles the innards of a lightly used kettle and the other will have a stale coffee smell that is not pleasant. In fact, both aromas are not exactly pleasant. The more pronounced stale coffee smell in the "clean water", then it's time to do the dirty business. Of course, older machines will have a natural odour that is not so pleasant even after rigorous chemical cleaning, e.g. it''s difficult to get it to smell like tap water, but this concept gives you an understanding of how taints are introduced into your cup and to some extent the level of rancid coffee oil buildup. For those of us with a more developed sense of smell, this is a very quick and easy test. Colour of the "clean" group water can also be a basic guide (specifically the amount of coffee particles), but more difficult to determine - needs excellent eyesight.......or a magnifying glass Enjoy !
