Hitting 429 Too Many Requests on /api/v2/users bulk update

Running a script to update 500 agents via PUT /api/v2/users/{id} and getting hammered with 429s. The exponential backoff logic looks right but the rate limit keeps resetting faster than the sleep interval.

var delay = Math.Pow(2, retryCount) * 1000;
Thread.Sleep((int)delay);

Is there a specific header indicating the reset window or am I just missing something obvious with the SDK client?