Every ATLAS response includes X-RateLimit-Limit, X-RateLimit-Remaining and X-RateLimit-Reset. Read them rather than guessing.
On the Standard tier the limits are 600 requests per minute and 200,000 per day. Sandbox is 1,000 requests per day against a fixed sample catalogue.
A 429 response includes a Retry-After header. The SDKs back off exponentially and honour it; if you are implementing directly, please do the same.