Viagra (sildenafil) typically starts working within 30 to 60 minutes after taking it, with some people experiencing effects as soon as 12 minutes. The peak effectiveness usually occurs around 1 to 2 hours after ingestion

. The effects of Viagra generally last about 4 hours, during which it helps improve erections when sexually stimulated. However, the duration can vary depending on factors such as dosage, individual metabolism, age, liver and kidney function, and whether it was taken with a fatty meal (which can delay absorption)

. Some people may feel the effects for up to 6 hours, and in rare cases, small studies have shown effectiveness lasting up to 12 hours, though this is not typical

. Despite the medication remaining in the body for up to 24 hours, its concentration usually falls below effective levels after about 4 to 6 hours

. Viagra does not cause an erection without sexual arousal and does not necessarily prolong the duration of sexual activity, only the ability to achieve and maintain an erection during its active period

. In summary:

• Onset: 30 to 60 minutes (sometimes as fast as 12 minutes)
• Duration: about 4 to 6 hours (effects start to decline after 4 hours)
• Maximum effect: 1 to 2 hours after taking
• May remain in the system up to 24 hours but with minimal effect after 6 hours

Viagra should not be taken more than once in 24 hours

. This information is consistent across multiple medical sources and expert reviews.