Personally, Hannah, I don't grow many flowers from seeds anymore , but when I worked at my friend's greenhouse the Petunias we planted reached maturity in about 3 months. They took anywhere from 7-21 days to germinate and the survival rate was very good, so we had plenty of hanging baskets and planters ready for Mother's Day.
You're welcome, Hannah! What color are you planting? Actually, I had thought about trying to plant some Wave Petunia seeds this year, but I have so much on my plate right now, I didn't think it was such a good idea. Maybe next year!