I have used this is my classroom for three years. It works like a charm. The students love to see how many gum balls they can earn during the given time. I have either set the timer for the full amount of work time or set it for smaller increments of time. As a reward, if they reach the goal, I sometimes give a piece of bubble gum, or something toward a classroom goal for some other reward...For example I might give them a letter toward building the word Compliments.
Once students enter the room each day, the timer begins. The timer stops once lunch count/attendance is taken on the Smartboard, everybody's materials are out on their desk with their binder underneath the desk, and everyone is seated. Sixth grade students love competing to beat their previous time. The best part? I don't have to say a word. My students encourage everyone to follow expectations and get ready to go as fast as they can.