To harvest seeds from a flower, wait until the flower stem turns brown. Planting the entire seed head is more likely to induce rotting. So take a ripe seed head indoors - 2-3 weeks after a flower is done blooming and when petals are brown and crispy - and separate the seed from the flower head over a paper towel. Some seed needs to dry thoroughly and others need to be sown right away. The internet has lots of info on growing plants from seed; search for your plant. Depending on the plant, some types of seed germinate rapidly and others take months. Seeds from annuals may not be winter hardy and must be stored inside, often in the refrig. Some seeds require special prep to aid germination. Unfortunately, the answer to your question is not straightforward. Another factor to consider is that hybrid plants to not come true from seed and some hybrids are sterile - produces no seed. This website has information at https://www.gardeningknowhow.com/ornamental/flowers/fgen/harvesting-garden-seeds.htm
