Well, if you wait long enough, it'll just decide to do it by itself! Mine does at least once a week, but then again, I give it quite a workout and have it bogged down with many widgets, lots of background apps, and a fair amount of WiFi activity.
However, if yours isn't rebooting by itself, the only time you really need to manually reboot is if you think that the N7 is getting sluggish or in some way not performing normally, just like a PC. After a while you'll get to know its "personality", and this will help you determine if rebooting is necessary.
Of course, if you're simply not going to be using it for more than a day, you might as well shut it off during that time.