If you only wait 24 hours it aint worth wasting your time going to spray it as it aint going to do anything. Round up pro takes about 2 weeks to kill weeds.
Roundup (and most) weed killers contain the active ingredient glyphosate which is systemic in action. This means that it is absorbed through the leaves of the weed and then it travels within the weed to all parts including the roots.
Once it has fully circulated within the weed, it then causes death....by preventing the plant making it's food.
Glysophate is bio-degradable it will break down instantly on contact of anything other than leaf matter. It is absorbed into the plant within about 4-6 hours....Hence no rainfall should occur for that period. The damage is then done regardless of outward signs.
Simon
I would advise you to do research on the chemicals you are using around peoples property's, family's and pets..