You don't necessarily have to take the whole dose in one shot. Viagra has a half-life (the time taken for your body to eliminate half of it) of about 4 hours.
I was on 50mg, but that wasn't enough (yes, I have a clinical need). 100mg does the job but hurts like hell, unless I take 50mg 2 hours before and another 50mg 1 hour before (with a standard 500mg paracetamol with each). I'm losing a bit of the earlier dose, but enough is left.
Interestingly, the symptoms of the first 50mg are more noticeable than those of the second, even though the concentration is higher after the second. It's as if the symptoms are triggered by the first one, then my body has gotten to grips with the problems (headache and facial flushing) by the time that the second one hits. Two hours after starting my 100mg programme, I'm feeling better than I did one hour after taking just 50mg.
The advice on the packet says you should only take it once per day, but as I'm not exceeding the recommended daily limit of 100mg, I don't think that matters.