I want to know when a DSN key rate limit change takes effect.

DSN key rate limit windows do not reset at the top of the next clock hour. For example, if your key has a limit of 1,000 events per hour, the one-hour window starts at an arbitrary offset rather than exactly at :00.

Because of this, reaching the next clock hour does not necessarily mean the rate limit has reset. For more details, see this GitHub comment on rate-limit period timing.