[azapa]

there was a few interesting posts recently from navy documents (bad memory here) indicating how exponentially much greater the risk of blackout becomes over 1 min downtime. if you are by yourself, limit to that. It is very impractical to say "don't dive alone" though, as we all, in some form or another do it. but, as all above say, 2 to 3 mins. down time, alone, and you'll die before the years done..