The obvious answer is simply to explain your circumstances. Keep in mind that most cases will be relatively simple, and there will be changes of pace (one witness called, another witness called, etc.) *AND* keep in mind that attornies want to keep you interested enough that you don't miss something important.

Many people called to serve never end up on a jury; I've been called twice, once for "one day, one trial" in Philadelphia, and once for a week in Ohio, and in six days, I only sat at a junk trial (one quickly declared a mistrial, due to the ineptitude of the plaintiff (who was representing herself)). As the court folks were fond of telling us (so we didn't feel cheated/used/etc.) many people, when realizing it's about to become "real", realize that a plea bargain is really not a bad idea, or that (in a lawsuit) the proposed setllement really is pretty close to fair.