Be nice. If you're sending too many requests too quickly, we'll send back a 429 error code (TooManyRequests).
You are limited to 60 reqs/min per account. Practically, this means you should (when possible) authenticate users so that limits are well outside the reach of a given user.
The API response headers also give you the following X-RateLimit value
|X-RateLimit-Reset||1406096275||The unix timestamp when the rate limit will be reset.|
|X-RateLimit-Limit||60||The rate limit ceiling for your account per min.|
|X-RateLimit-Remaining||59||The number of requests left for the 1 minute window.|