What are permissible requests "speeds?"

The API will cut you off if you make more than 30 requests over 5 seconds to any single endpoint (currently 30 requests per second, https://api.stackexchange.com/docs/throttle).

We consider this a breach of the API interface contract, so you won't get a nice error back. The exact manner in which this constraint is enforced can (and will, in all likelihood) vary over time. Attempts to work around this limit will be consider unconscientious use of the API.

Note that you can stay within this limit and still blow through an entire [app]s quota in less than 1/2 an hour.