It depends upon how mucky it is. I like to keep it clean, but I'm not a slave to it.
The degree of cleaning also varies. It can be as simple as knock off the mud, all the way to a full on deep clean and a polish. That last one will also depend upon how nice the weather is while I'm doing it!
I ride twice a week and during the winter slopfest I'll clean the drive train and brakes after every ride and clean whole bike every other ride which will be the Sunday morning one so at least it's still light when I'm cleaning it.