You will know when to use a belt during your first few lifts. Everyones diffrent, use it when necessary on heavy lifts. As you lift more weight your back will get stronger and get used to each level of weight. And like some peolpe have mentioned, some people dont use a belt. Look at DR.Squat when he was the 1st to squat over 1000lb! check out the video if you havnt seen it:
http://youtube.com/watch?v=fivhv5znPg0
Look carefully his belt isnt even tightened its hanging loose. Now that one hell of a strong lower back.