Since none of my wife's uncles were stil lving, we tested 4 of her known 1st cousins to see if we could uncover the mystery. One of my wife's male 1st cousins is now showing as a Close family - 1st cousin match and I suspect that he and the new cousin are really half-siblings.
Your healthcare provider will tell you how much VIAGRA to take and when to take it. Do not take VIAGRA more than once a day. VIAGRA usually starts to work within 30-60 minutes. You may take it up to 4 hours before sexual activity.
There's no set answer for your question but I can tell you this. Taking the pill does not give you an erection... it ALLOWS you to achieve one through sexual stimulation. For me, less than 30 mins after taking a 100mg Viagra it's in my system enough to get an erection. Also, I've been able to get an erection 16 hours after the one pill was taken. But then again, it's 100mg I've been prescribed.