How often should the separator roller be cleaned?

The Separator roller should e cleaned whenever the scanner is cleaned.  The sticky sheets used to clean the transport rollers will keep the roller in fairly good condition.  If the scanner seems to have a number of multi-feeds, it is a good indication that the separator roller will need to be removed and thoroughly cleaned.  See FAQ “What is the best way to clean the separator roller?” for the best way to clean the roller.