I have my solar panels cleaned once a year. I live in an area that is fortunately not particularly polluted as there is little development between me and the ocean. The winds are always from the ocean, so the panels do not get particularly dirty. I did not clean them for the first four years as I simply forgot about the need. I did notice a big improvement in power generation after the first cleaning about 20-25%.
For the last couple of years, I have been cleaning them yearly in the beginning of spring. With PG&E it is very difficult to get good data on how much power the panels produce. I am thinking once a year is probably the right amount of time between cleanings, which cost $125.
I just had mine cleaned by the same person who had just cleaned the panels of Martin Eberhard, who was the former Tesla CEO. The solar panel cleaning man is a talkative fellow.