At least once a day. I always weigh myself first thing in the morning after I've gone to the bathroom so it will be the lightest/most consistent weight. I will also weigh myself later in the day out of curiosity, but don't count that number. Days where my weight goes up or stays the same are discouraging, but days with even a tiny loss are awesome and keep me motivated.
I've tried to do it just once a week, I even put my scale in the back of the top shelf of my linen closet to hide it from myself. The next morning I was climbing up there to dig it back out. lol