This varies per person. Usually it takes about an hour for it to kick in-- but even that varies.. Some men swear it's a 24 hour period- others have mentioned to me that it really works best for them for one sexual act in the three to four hours following when they take it. This also may have something to do with dose.. some men work fine on half a dose, others take the full pill...
Why don't you experiment with the drug- spread the time you try to have sex one hour at a time and see how long it lasts for you-- if it is not enough, ask your doctor about the other erectile enhancers- --one of them may work longer for you.
I tend to find that if i take a pill (i only use 25mg) at 8pm...then i can guarantee that by 8am it is still in working order..
i must admit that i havnt tried it further than this
Obviously with a higher dosage it might last longer.
Michael