I grow my zinnias in containers as well. Someone had given me a kit that came with a packet of seeds, growing medium and a tiny clay pot. The pot was maybe two inches so I only started one or two seeds. I put the rest of the seeds in a pot outdoors. The magic growing medium sprouted the seed almost immediately and the outdoor seeds took a week or two, I forget. Anyway, they were extremely tall, too tall for their container. Because of that, I recommend planting your zinnias in a container that has some depth if you're not putting them in the ground.