I've never cleaned our fog machines
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.