If you go to Stokes they say the Wave Petunias are day length sensitive.
http://www.stokeseeds.com/product.as...CategoryID=456
The instructions for Grandaflora Petunias
http://www.stokeseeds.com/product.as...CategoryID=431
show March 1 for June 15th flowers. That means that if you planted them about now you'd have flowers around July 4th.
The wave petunia seed sure is expensive.
What I have done is experiment with cheap seed until I am sure of my technique before I try the expensive seed. I've having problems with loosing too many after they germinate. I suspect that it's a light problem.