I harvested petunia seeds this week. Can I grow them now ( in July) in Toronto and get flowers?
Excellent idea to be saving seeds! But I am afraid you will be too late for flowers this year. Normally, you would start the petunia seeds in a growing medium indoors and it would take 7 to 10 days for them to germinate. Add another 9 weeks or so for the seedlings to be strong enough to plant outdoors and now we would be in mid-September. It would normally be another few weeks before the plants are mature enough to flower and by then, the days would be much shorter and before long, frost!
So, save the seeds til next year and sow them in pots in late February or early March. Just transfer the perfectly dry seeds to small envelopes or lidded jars for storage in a cool dark place until then.
For a full explanation of how to grow those seeds into flowering plants successfully, see the Iowa State University fact sheet here.