Half-life isn't easy to say, but it isn't terribly relevant to how long the effects last. Different strains last for different amounts of time. I'd say 2-8 hours is the range for perceptible effects. The drug certainly sticks around for longer than this however. The exact half-life I do not believe is published.
The size of a dose for any given person depends on their opioid tolerance. For zero opioid experience and an average sized male, I'd say about 4-5g would be ideal, though with certain strains this could be too much.