A simple answer for the OP: a good rule of thumb is a 4:1 glide ratio in a helicopter (not perfect but close enough). So to glide 1 nm you need 1/4 nm, or roughly 1500', of altitude.
Given the use of terms, like 'impact' and 'hopefully surviving' I assumed you do not fly helicopters. Some glide better, some worse, but the biggest deciding factor will be pilot actions.
Last edited by pilot and apprentice; 10th Jul 2013 at 04:06.