Tadalafil's half-life is 17.5 hours (meaning that 50% will be out of your system by then), but can last 24-36 hours. PT-141's sublingual half-life is unclear, but is estimated at 6-8 hours, and effects also last up to 24-36 hours. This is why daily dosing tends to work well and maintain the ability to be 'ready' at all times.