How often do you wash your roof?

I have a small RV with a silicone roof. I do a full inspect and wash maybe twice a year with a terry cloth pad, tad bit of car wash soap in a spray bottle and just hose water to restore it to a gleaming clean surface. Otherwise a hose (and maybe a rag) is usually enough for regular cleanings, which are done as needed. If I'm getting regular rains, it cleans itself, but if I have multiple Haboobs rolling over me, I clean much more often.

I guess it all depends on your roofing material and the conditions that roof is under.