It may depend on other factors, so for me a fit and healthy 62 year old the effect lasts around 4 or 5 days. I do sometimes just take a half tab and get a couple of days with that. I'm lucky I don't get any headache, but do feel a bit off for the first 2 or 3 hours and get a stuffy nose, as if a cold were developing. As others say, drink plenty of water during the first few hours.
For me now Cialis is magic and without it I would struggle to keep a good erection long enough, with it I perform better and longer than I did as a young man. Thank god this was discovered by the time I came to benefit.
Might it also depend on your weight and what about the quality from the supplier?