The Champions Online Website outlines the schedule for open beta testing of Cryptic's superhero MMORPG, and how the GameStop early access will work:

We’re pleased to announce the schedule for next week’s Open Beta! On Sunday, August 16th, all characters from Closed Beta will be wiped from the servers in preparation for our Open Beta. When the servers open at 10 a.m. PDT on Monday, August 17th, everyone will have a fresh start, and you’ll be ready to experience the game from start to finish!

Servers will be taken down nightly from 1:00 a.m. PDT to 3:00 a.m. PDT for scheduled maintenance. Other than that, you can play all week long until the 24th. At 10:00 a.m. PDT on Monday, August 24th, the servers will be shut down and wiped once more in preparation for the game’s release. The “Early Start Program” begins on Friday, August 28th, and allows select pre-order purchasers an opportunity to get a headstart on the rest of the world by playing the game a few days before the official release. In addition, all Early Start participants will get to take part in a world event, where special enemies will drop unique items which will only be available until September 1st.