Once a year is ideal, it is also highly recommended to have your solar checked independent of the installation company by a reputable, qualified solar inspector in the first 2 years of it being installed.
This keeps the install company honest and ensures they can be held accountable whilst they are still in business for any issues identified by the inspector.
It is important to understand your solar system has many parts that are reliant on each working right and there are many potential causes of failure. One way you can look after your solar panel system is to get it serviced at least once a year, 2 at the outside.
Again, just like your car, you wouldn’t drive it around for 2 or 3 years without getting it serviced.