Viagra typically takes about 30 to 60 minutes to start working for most people. The effects usually last for around 4 to 6 hours. However, the onset and duration can vary depending on factors like the dose taken, body weight, age, existing medical conditions, and whether it is taken with a heavy or fatty meal, which can delay its action. Sexual arousal is still necessary for Viagra to work effectively, as it helps improve blood flow to the penis but does not cause an erection on its own.