I have sometimes accidentally left a tampon in for over 8 hours, and I've been fine. I'm pretty sure that TSS occurs when you take out a tampon that isn't saturated yet, or is pretty dry (sorry about TMI here), and so when you pull it out, it takes off a layer of skin from the inside of your vagina as well. Then when you put the next tampon in, you could have an area where the bleach from the tampon can get in and cause TSS. I'm pretty sure that's what happens. Of course, if you leave it in too long, it can get pretty yucky and can also cause TSS. I guess it would depend on how heavy you are - I get very light periods and so I never really seem to have any problems, although of course I always try to remember to change them every 5 hours.