Review Of How Long For Roundup To Work Reddit References. Roundup takes anywhere from 12 hours to 7 days to work, depending on the product. Roundup takes anywhere from 12 hours to 7 days to work, depending on the product.

This rapid absorption means that roundup is resistant to being washed away by rain in as little as 30 minutes. The issues we're seeing are with people who use this stuff on a daily basis over the course of many years. Most of the new formulations claim 30 minutes.