According to Xero Developer Help Center, there is a daily limit of 1000 API calls that a provider can make against a particular Xero organisation in a rolling 24 hour period. If you exceed this rate limit you will receive a HTTP 401 response with the message “oauth_problem=rate%20limit%20exceeded&oauth_problem_advice=please%20wait%20before%20retrying%20the%20xero%20api” in the http response body.