10 days of hair growth would be enough time to shave. 3-4 days to let the thc stop depositing and roughly a week to get new clean growth away from the scalp. Now this is exceptable for head hair, but body hair doesn't grow quite as fast. What you really need to figure out is where you want them to take the sample from. If I were in your shoes I would let my head hair grow out because its going to grow faster and given your time frame you may be pushing it to get 3\4 of an inch of growth.