That depends on the length of the growing season, the amount of rain, and the particular species of weed you're talking about. They can range from millimeters to yards.
If that's not the kind of weed you had in mind, too bad.
First, let the weed crimple and die completely. Then, Rake them up and throw away. note- If using the correct weed killer they should be very easy to rake up.
It kills their roots. Answer 2 Not necessarily their roots. Some weedkillers (paraquat, for example) just kill the green parts of plants. Obviously the roots may die in the process but only because...