Interesting question! I've been using tampons since I was 13, I do not like pads at all. I have a light flow, so a regular or lite tampon for me will last anywhere from 4 hours to 8 hours. I have heard some women say that they have to change them in 2 hours because of a heavier flow.
I guess what I do is give the string a very light tug, a full tampon will slide out easily. A mostly dry tampon will not move much. A full tampon can allow some leaking, also. If I discover leaking when I wipe, I change my tampon.
I guess I would suggest during the first period you wear tampons, be tracking how long it's been between changes, for next time.
As long as you change tampons reasonably often, like 8 hours or less the chances of toxic shock are extremely small. Almost all the cases of toxic shock caused by tampons occurred with the Rely tampon, which was taken off the market. That one was so absorbant that women left it in sometimes for days. It actually was shown to leech the moisture from the inside walls of the vagina, causing skin damage and increasing the chances for infection to take hold. It was made from a different material than the tampons on the market now.
http://en.wikipedia.org/wiki/Rely_(brand)