The recommended periodicity of trademark monitoring will depend on your jurisdiction or, more precisely, on the length of the opposition window in that jurisdiction. On average, opposition windows tend to be open for 2-3 months, so performing monitoring once a month should suffice. However, in the USA, for example, the opposition window is only one month long, which can lead to you not responding to an infringing trademark application in time.
The market standard for trademark monitoring services is to send monthly reports. At Trama, we offer trademark monitoring with weekly reports, so you'll always have enough time to look into the matter and respond.