I pop 50mg about 90 minutes to two hours before a punt. I make sure that my stomach is empty because food in it can slow down the absorption of viagra. I always know when the viagra kicks in, usually about 45 minutes to an hour after I take it, because for a few seconds I get a very light buzz, a floaty feeling, nothing to make me fall over or drive dangerously, just an unmistakeable light headedness that lasts maybe 5 seconds. The effects of the viagra will last around 5 or 6 hours, so I have four hours in which to punt, usually plenty of time to go to plan B if something happens to plan A.
In my case, at 67, it isn't nerves, its age. I can still get very hard, but maintaining it for a long period is long gone.