Always twice a day – once in the morning and once in the evening. No more no less (over-washing can have the opposite effect). While you might think that pollution can’t get to you inside while you sleep, dirt and bacteria can.
Washing your face first thing in the morning will get rid of any impurities that have gathered during the night so you have a clean slate when you head outside. If you don’t, you’ll have twice the amount to wash off in the evening which not every face wash is cut out to do. You’re almost guaranteed to miss some.