One and a half min to about four sounds about right. I can normaly get somebody from 30seconds to about 2min 30 in about 2-3 hours in a relaxed enviroment. Apnea walking is still the best training aid as far as Im concerned. Just find a nice soft field and walk as far as you can holding your breath. Give your self three times your walk duration as recovery and see if you can go a bit further.. DONT HOLD YOUR BREATH WHILST DRIVING!!! good luck.
