Petunias typically bloom from mid-spring to early fall, with the exact flowering season varying depending on the location:
The Petunia flowering season is a marathon, not a sprint. For hanging baskets, borders, or ground cover, you cannot beat their longevity. Just remember two things: full sun (6+ hours) and regular fertilizer (every 2 weeks). Do that, and they will reward you with color from the last frost to the first snow.
In USDA Zone 6b, my petunias start their show in mid-April (if started indoors or bought as mature starts) and do not quit until the first hard frost in late October or early November . That is nearly seven months of continuous blooms. Unlike many annuals that take a "rest" in summer heat, petunias seem to hit their stride in July.