I think it lasts one game hour [one minute IRL], maybe two. Feeding does increase the timer, and no, you cannot force yourself to change back.
Here's some perks though: You can sprint faster than any horse, your carry load is increased by 1900 points, and power attacks have a really fun knockback effect.
I did notice that knockback affect. I was having fun killing some Silver Hand and just back handing them across the room. It was hilarious.
So, the increased carry weight is that only when you are a werewolf or across the board? I did notice that when I'm a werewolf I can't "activate" doors and such. Or can I? I also noticed that whatever equipment I had equipped at the time of my change remained equipped.
I tried waiting for an ingame hour and I didn't change back. I ended up waiting 4 hours before I changed back. But I did feed three times. So, I'm assuming that it's an hour by default then an hour each time you feed. So, if I kill eight people that means I'll be a werewolf for 9 hours. Does this sound correct? I should also note that I use; "Set Timescale To 9" when I'm playing so my days and time are much longer.