Bumping this up! I'd love to know the answer, too!

Well most of my zinnias have off shoots or areas where there looks like another growth shoot would form out and grow another flower out. I cut mine off about two leafs sets down or to where you would see a new set of leaf shoots coming off the stem, because you should probably get new flowers from that point. If you only have a single stem zinnia, I would say cut down stem to leave two sets at the very least one set of leaves left on the plant to see if it branches and buds out from there. If you find u have a short variety, say cut and come again, every time you cut your flowers, you should get more flowers and get shoots coming off the plant for more flowers. Hope this helps?