OK will doTake a photo at f22 to see if you need cleaning.
There is no required frequency as such.
from what I remember if you do get it on the sensor, if it is left for a length of time, it kind of bakes on.I've never noticed any muck on mine, but I do usually shoot with large apertures. But I probably should check it though, seeing as I got my older 450D 3.5 years ago!