Although the label might say it's ok to plant again after 2-3 days, do note that different products have different time frames and you should read the label for your product carefully.
Even then, you shouldn't trust manufacturer's claims. This article cites a 1993 EPA study showing that glyphosate (Roundup) half-life can persist in agricultural soil for over four months. (Scroll down to figure 6 and the section titled "Persistence and Movement in Soil".) And that's just the half-life! Glyphosate does not break down quickly.
Note that Monsanto has been in trouble in both New York and France for false advertising regarding Roundup biodegrading in soil and exaggerated safety claims.