Solar arrays are valuable revenue generating assets that require a certain amount of maintenance. How much exactly depends on what type of installation we are talking about (ground mount vs. roof mount), the size of the installation (obviously larger systems require more than small systems), the system's age (older systems require more), and numerous smaller factors such as the tilt of the panels (flatter systems require more maintenance) and the quality of the original installation itself (better installs should lead to less maintenance).
With all of these factors influencing the amount of maintenance a system requires how is the system owner to know what is appropriate for their system? The easiest way to answer that is by asking what your risk tolerance is. If you are the type of person who gets their oil changed every 1500 miles, and never misses a checkup at the dealer, then you should definitely have your solar array looked at more than once a year. If you tend to push the envelope when it comes to changing your oil or painting your house, then maybe an annual checkup is enough. All systems, no matter what their size, tilt, mounting method, etc. should have an annual check up performed by a qualified technician.
The best time to perform maintenance is actually in the late fall / early winter. Daily generation rates are at an annual low at this point on the calendar and shutting the array down for a day or two if necessary will not amount to significant lost revenue. In fact sometimes it is three times cheaper in terms of lost revenue to maintain a system in December than in the peak generation months of April or May.
Some other points to consider about solar maintenance: