Both the Pinboard & Instapaper APIs reserve the right to rate-limit callers. In the case of Pinboard, the advertised acceptable rate for most endpoints is one request every three seconds (much worse for a few selected endpoints), but the author has seen far better than that in the wild. The docs suggest: “Make sure your API clients check for 429 Too Many Requests server errors and back off appropriately. If possible, keep doubling the interval between requests until you stop receiving errors.”
Instapaper is a bit more coy, only alluding to rate-limiting in their documentation for a 400 response code as being returned for “a bad request or exceeded the rate limit”. The rate limit is never defined, and the author has never encountered it in the wild.
Regardless, this implementation will take into account the possibility of rate-limiting by retrying on certain status codes, halving the request rate each time.