The half life of cialis is 17 hours, so one day after you take a dose the levels in your blood will be more than halved.
So it might work up to 36 hours or more for some, but if you have true ED it is best to take it once per day to keep the levels even. The dosage is dependent on your needs.
Try 20mg for first day and 10mg thereafter.
Again, if you don't have ED then you might find that even a small dose has a noticeable effect for days.