Your add-on can make 500 API requests per 5 minutes. Once you exceed the limit, calls will return HTTP status 429 and a message telling you that you've been limited.
Additionally, API methods that involve sending messages to rooms or individual people are limited to 30 requests per minute as a means of preventing room "spam." This limit is room and person specific. Messages sent to one do not count against the limits for other recipients in your group.
Please let us know if you run in to any trouble -- we're pretty flexible.
Your current rate limit usage is returned in the response headers from each request you make. For example, for the overall resource limit:
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HTTP/1.1 200 OK X-Ratelimit-Limit: 500 X-Ratelimit-Remaining: 499 X-Ratelimit-Reset: 1366037820 ...other headers not shown...
For the room specific notification limits.
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HTTP/1.1 200 OK X-FloodControl-Limit: 30 X-FloodControl-Remaining: 8 X-FloodControl-Reset: 1366037820 ...other headers not shown...