We implement rate limiting to make sure the API is responsive for all customers. The rate limits are set in a way to provide substantial functionality while trying to stifle abuse. Most times, a rate limit can be subverted by implementing a different API call or leveraging other API features (i.e. instead of polling for quotes, leverage the streaming API).
Each limit is enforced by the minute and on a per-access-token basis. As such, the limits are enforced on per app and per user basis.
With each request that has a rate limit applied a series of headers will be sent in the response. These headers should help you to gauge your usage and when to throttle your application. For example:
X-Ratelimit-Allowed: 120 X-Ratelimit-Used: 1 X-Ratelimit-Available: 119 X-Ratelimit-Expiry: 1369168800001
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