Most pellet grills use up 1–3 pounds of pellets per hour. If a bag comes with 20 pellets, it follows that you will be able to use the grill for about 20 hours approximately. If you make your own wood pellets, it is likely that you always end up making them in excess. You should keep wood pellets in plastic bags or woven bags so that you can prevent them from coming into direct contact with air and moisture.
If the humidity of the environment in which you store the pellets is less than 10%, you can preserve the wood pellets for 6 months.