The option in the launcher says “Beta & Stress Test: WoW Classic” but if your invite is only for the stress test, you’ll get access today and up until tomorrow 6pm PDT when the stress test realm goes offline. It’ll come back online whenever the next stress test is scheduled, and your access will be back then as well.
The window 4-6pm PDT today is just when Blizzard will actively be monitoring the realm, thats why they want everyone possible during that time. But the realm itself will remain open until Thurs 6pm PDT.