I am not sure what exactly effects the amount of time buddy aid is given. I usually play Elite and the time varies. It seems like the morale, suppression state, vicinity of enemy contacts all play some kind of effect. I noticed troops giving buddy aid for several turns to one man and others give buddy aid in 30 seconds. Dead are always quicker as your men are presumably just picking up ammo, weapons, and the poor saps letter to his girlfriend. I do not really think buddy aid is worth the boost in score you may receive. But I make it a point to try and rescue any wounded soldiers. It is fun to try and win and at the same time rescue your men. All part of the game.
Also, sometimes seeing where all your wounded soldiers are lying is pretty hard. Click on your top tier HQ to highlight all your troops (even the ones lying on the ground bleeding to death) and then go and help them out.
