Time-turners

What do you know about time-turners?

I'm trying to write a time-turner fic and I want to get it as correct as possible. That means I need to know what is canon when it comes to time-turners and what I can "claim" is the deal with them in my fic :)

For instance, if a person gets thrown back 20 years back in time. Will that person need to stay in the "time-turner"era for 20 years or is there any way to get back before that?

Does the time-turner "know" that you are going to use it, and the events that you cause will already have happened? (Like when Harry cast the stag-patronus the first time?) I'm a bit confused by the whole thing, but would really like to try to write this fic :D

Tell me everything you know is canon, and if you have theories of your own or have read some good time-turner fics I would appreciate that too, just let me know what is canon and not :D

I hope I get a ton of comments on this one :D 

First draft for the fic is due February 7th and posting begins in March