how long can you keep a "try"?

hi,

I want to try Dreamweaver..  and I need to know, when you try a product, how long from the time you start the try to when they start charging you?
I suppose they don't send you a warning a before they charge you so you have a chance to cancel the "try" before they start charging you..
so: for how long is it SAFE to keep a "try" program?

I found two examples, here and here, of things gone wrong..

thank you...