The event/promo runs from May 1 - 10, from 3 to 5 PM. Starbucks Rewards members get an extra hour; for them, the event runs until 6 PM.
In my area, a Grande Frappuccino is normally around $5 varying with flavor (you can go for any size for the promotion), although the price can go up a fair bit once you start going crazy with the syrups/sauces.
If you're looking to try something new, they just released the new S'mores Frappuccino and have also brought back the Caramel Ribbon Crunch Frappuccino for a limited time.
Photo via Starbucks.