Felly,
1. Do most people pray out loud, whisper, or just think the prayers to themselves?
I believe that most folks have a higher power that accepts just about any form of transmission.
2. Can you pray while doing other activities or should you do it old school (kneeling.)?
Anything important that I attempt pretty much requires my concentration to get it right. That being said, I was once told by a fellow alcoholic (a Catholic Priest) that praying is 37% more effective if done on one's knees. Sounded good to me, plus the act of kneeling gets me in a proper frame of mind. I have however been known to pray in just about any position if guidance was required ASAP. I have found that prayer is more effective for me when I ask for DIRECTION and not SPECIFIC THINGS. The very act of asking puts me in mind of the fact that I can't do it alone.
I do meditate as was previously said in order to get the answer that prayer is asking for.
3. How long do you usually pray for at a time?
Not one second longer than it takes to get my problem laid out to my Higher Power. Based on my confusion this can be seconds to many minutes. If it takes longer than that I probably haven't done all I can do before passing it off to the higher power. My higher power seems to prefer that I do my part before I come knocking and if I do it does seem to shorten the length of my prayers.
All great questions and the fact that you are considering them speak well for the path you are following.
Good luck and remember, there isn't any right way, just one that works for you.
Jon