Of course there isn't any particular "how long." Just do it for as long as you do and then repeat as soon as you notice you drifted off base. Succeeding isn't the point. Its noticing that you drifted and refocusing.
Yep. This is where a personal mantra can be helpful. Everytime I catch myself spiraling down a thought stream I repeat "empty". Over time association will help you get into a meditative mode much more quickly.
If you fall asleep change your practice time to when you are less tired and make your sitting position less comfortable.
Also meditate as early in the day as possible; you want to be fresh. Plus you will benefit from its immediate calming effects the rest of the day.