How long after I plant the seeds can I expect to see petunia flowers?
Petunias are normally propagated from seed. Petunia seed is best sown indoors four to six weeks prior to planting outdoors. Sow the seed in a moist, well drained, pasteurized medium. Commercially prepared mixes work best to germinate seed. Do not cover the seed when sowing. Water with a fine mist from above or sub-irrigate from below by setting the germination container in a shallow container of water. Excessive water should be allowed to drain. The germinating medium can be kept from drying out by covering the container with a pane of glass or with a plastic bag. Keep the container in a location with bright light, but not in direct sunlight, until germination begins. Maintain a minimum soil temperature of 70 degrees F and maximum of 80 degrees F. Seedlings can emerge in three to four days under optimum conditions. During germination, try to avoid conditions (seedlings being too thick, excessive soil moisture and cool soil temperatures) that favor damping-off disease. Once the seedling
From personal experience most petunia flowers will bloom in around 3 months if started from seed. This can still depend on the weather of course and the amount of water they receive. Sources: http://www.seedman.com/petunia.
