Everyday! Everyone should be cleansing their face once in the morning and twice at night every single day, no exceptions.
Believe it or not, your skin can accumulate environmental pollutants in your sleep, so it's necessary to wash them off in the morning as well. Dead skin cells and pollutants may collect on your pillowcase and be transferred to the skin throughout the night. On that note, it is essential, especially if you tend to have an oily skin type, that you wash your pillowcase regularly!
Now, at nighttime is when the skin is likely to be covered in makeup and grime, so for your night cleansing you do need to cleanse twice! Do not go to bed with makeup on - ever! Your skin will become a breeding ground for bacteria.
The first cleanse removes the makeup and grime accumulated during the day, and the second cleanse flushes the pores and allows the skin to breathe.
Always double cleanse - every single night. I swear by this.