If its 40 percent,6 and a half hours is a good time to go by.that is if your 170 pounds or over,you can not be asleep during that time either.you must be awake and active no matter the bad headache/mood you aquire in the last 2-3hours.if you have a blow and go you should be good.if you weigh less look at the charts and figure it out,otherwise you will pay dearly.try not to cut it close always add at least 20 minutes to what ever formula the chart says,eat well get active.don't test that dam thing cause you will fail.