I've personally only owned two fog machines and I've never cleaned either of them. They get used durring halloween and speratically through out the year (possibly 2 or 3 other dates spead out over a year) I just add new fluid to it everytime I use one. Generally they get run dry and I dont check them before I put them up. The haunt I volunteer at has gone thru a few just about every other year due to poor storage conditions or damage during the haunt season.
I did find this interesting post on a theactrical site though...
Hope it helps.