How long does the Steam "too many activation attempts" cooldown last?

The cooldown is exactly 1 hour. However, if you attempt to activate any key during the cooldown — even a valid one — the timer resets back to a full 60 minutes. Set a timer and do not touch Steam until the full hour is up. The limit is triggered after approximately 20–25 activation attempts in a short period. Failed attempts (invalid keys, duplicates, games you already own) count toward the limit and can lower the threshold.