Allow some zinnia flowers to mature and produce seeds. They may self-sow and grow new plants in the next season. Choose ...
Flowers: Marigold: 5-7 days, Sunflower: 7-14 days, Zinnia: 7-14 days, Petunia: 10-15 days. If you're seeds do fail to germinate, don't give up, keeping trying until you get it right.