Washing pillowcases once per month or every few months is usually good enough to keep the pillowcases clean.
Some people may want to wash pillowcases every week or every few weeks and that's even better to keep the pillowcases clean.
If you drool a lot at night then you might want to wash your pillow cases every week because laying your head on even your own drool is not a very good feeling.
I have to wash my sons pillowcases every few days because he drools a lot in his sleep even though he's only 8 years old.
I like to make sure his sheets, pillowcases etc are clean so his bedding gets washed more often.