How long do I need to wait for the flooding icing to dry before I can put a different color of icing on?
Depends completely on the look you want to achieve. If you want the top icing to settle into the flood (wet on wet) or to marble the two colors together, then you would apply the second icing almost immediately. If you want it to sit on top, then you'd wait for a crust to form at least, which can be a few minutes to several minutes depending on the thickness of the flood icing to start and your ambient humidity.
Thank you Julia!!!! I am going to try flooding some football sugar cookies tomorrow. I will post pictures after I am done. Thank you again.
Just beware that there's more risk of dark colors bleeding into light ones if you apply them wet on wet. In that case, I like to push my colors as thick as they can possibly go for the task at hand, as the thicker the icing is to start, the faster it will dry and the less time it will have to bleed.
Good luck with the footballs; I look forward to seeing the result!