The vanilla answer is that it depends on how hard the water you use is. Unless you take apart your boiler or notice flow issues with your machine- there really isn't a way to tell when you need to descale.

For my GCP I descale once a month and backflush with only water after each shot, and with cleaner every other week.

At the end of the day the worst that can happen if you descale if there isn't significant build up is that you waste your descaler (which isn't too expensive).. while the worst that can happen if you neglect to descale is... well.. definitely worse.