1. It depends on the medication. The one I take lasts six ours. When time's up, you take another (if you need).
2. No, haha. You'll feel sleepy, I bet you'll sleep. Sleeping is the best thing to do in a plane, time flies!

My medication is called Dramin here - If I'm not mistaken, it's called Dramamine. The official name is dimenhydrinate.

Good luck!