Tethering 4S, anyone?

Been thinking about activating the mobile hotspot option on my phone to use with my MBP and I have a Kindle Fire bc of the price and it was there when I stopped at Target (can't afford an iPad right now but the Fire is a great piece of iPad rip off) figure the tethering will come in handy for people with a iPhone and wifi iPad or other tablet.

So the question is from the point I activate it does it charge $20 for 30 days of service and I can notify AT&T to stop it before the months out? or keep going. Does it really only cost $20 (for how many devices)...I know this info is pretty readily available (although I didn't find what I needed so I came here) but I want "in the field" info