The best time to go to Santa Rosa Beach is October, when crowds are away, prices are reasonable and the climate is temperate.
In October you will pay almost half of what you would pay in June. Also expect a nice sunny and dry weather, with temperatures in the mid 70s.
Summer is the most popular time, especially June. In June, expect to spend more than $200/night for a hotel room in Santa Rosa. During this time of the year the weather is very tropical like, with high temperatures in the 90s and regular rains.