We’ll be actively monitoring the server, sending server messages, etc. during the 4:00 – 6:00 p.m. PDT period and it’s important to have as many people as possible log in during this window! The stress test realm itself will be available until Thursday, May 23 (becoming unavailable around 6:00pm PDT). This is from the blue post at the top of the Classic forum. Hope it helps.
https://us.forums.blizzard.com/en/wow/t/wow-classic-stress-test-1/178282/2