https://techcrunch.com/2018/11/02/world-of-warcraft-classic-demo-limited-to-60-minutes-of-playtime/

Put away the Jolt, Blizzard is limiting the time gamers will be able to play the World of Warcraft: Classic demo. Basically, after playing for an hour, players will be logged off and will have to wait 60 minutes before resuming for another hour. The goal is to ensure a mass of players do not crash the servers, which, honestly, if the services crashed randomly, would be the most classic thing Blizzard could do to recreate the original WoW experience.

In a forum posting Blizzard says it hopes to lift the session limits as soon as possible.