Generally as i recall with roundup, you spary, then you might leave for maybe 2 weeks to reassure yourself that its going ok, then you should be able to do what you want.
in theory AFAIK because its only damaging to green foliage and not to the ground, it should be safe to work on the area with 24 hours.
This is of course NOT the case with Pathclear and other such total weed control sprays. They work by killing the ground for upto 6 months. Products like pathclear literally kill the plant overnight but also kill the soil. Roundup has the impression of working far slower but has the benefit of not harming the soil.