For performance's sake, requests to the API are throttled.
As a rule, if you're making more than 30 requests over 5 seconds you risk being blocked. Registered applications can make up to 10k requests a day, per IP.
Be aware that IP sharing (multiple applications on a single machine, or using a cloud hosting service) can impact an [app]s throttle limits. Thus, we suggest always making the absolute minimum number of requests needed to perform a function.