I don't really keep track of it but I tend to clean my powerheads pretty frequently since they tend to get covered in slime algae. I'd guess once every few months. My skimmer pump gets clean a little less often.
HMMM I can't really choose as I clean different things at different times. My skimmer cup and neck get it about once a week if I remember or if it's bad. The skimmer every few months. Everything else get's cleaned once a year if lucky. I will siphon the sump though when I do a water change.
When I say thorough cleaning I mean actually taking the skimmer out of the sump and disassembling it and cleaning it thoroughly. Taking the pumps and running them in a bucket with vinegar...
I clean my powerhead once a month, with tap water and toothbrush.....I clean my skimmer once every 2-3 months....My HOB powerfilter gets cleaned every 3 months as well. I have a filstar that I use for carbon and filter floss that gets cleaned every 3rd water change.
I clean my hob filter with surface skimmer every 1st sunday of the month when i change water. While i'm cleaning that i change out the carbon and rowaphos. Other than that haven't really cleaned anything empty the skimmer cup every week and clean the cup.
I clean my collection cup every other day with a tooth brush, drain skimmate every 3 days, clean the pump every month. (you don't want the RD pump to die on you )
I clean the tunze pump every month with deltec EZ clean, quick rinse every week during water change by cover the outlet nozzle for 10 sec.
I also do WC thru my sump, which cleans out all the silt in there. I have a coral near the top of my tank that doesn't like to be exposed to air at all, so I can't do WC from the tank. Other than the skimmer cup, the rest I clean when needed, which is not very often--about every 6 months.