1. You don't completely go in to a posessed like state but if you get too wasted you may have a lot of trouble walking and talking and such yes.

2. If it's your first time (at 15? Christ...), you should notice it after 2 or 3 beers max, but don't let that stop you drinking 20.

3. It lasts as long as how much you drink. It takes an hour to completely process and release one unit of alcohol, there are two units to a pint of beer.

4. It's not hallucinogenic or sense modifying like cannabis, mushrooms or LSD, you'll get a stage of "tipsyness" or "merriness" which is pretty much the only good part if you think about it....then you'll start losing control of walking straight etc., and ultimately you'll puke it all back up, but it completely depends on the person as to how much you can hold. If you drink too much the majority will then get a hangover in the morning which will most likely consist of a dry mouth, headache and possibly some level of nausea.