It’s 15/180 requests per endpoint per 15 minutes per user per app, i.e. it drills down to the particular user of the app, and for that specific endpoint.
For example, statuses/user_timeline allows 180 requests per 15 minutes, and statuses/mentions_timeline allows 15 requests per 15 minutes; if you have 10 users, each of those 10 users can max out the limit for each individual endpoint over a 15 minute period. If you’re hitting a rate limit, you’ll be hitting the one for that specific endpoint, use of others won’t affect it.
The 15 minute window is not global, that too is per user and on an endpoint-specific basis; that is, they don’t all reset at, say, quarter/half past/to the hour. If you start to use statuses/user_timeline at 12:00, it will reset the limit for that endpoint at 12:15, while if you start to use statuses/mentions_timeline at 12:05, it will reset the limit for that endpoint at 12:20. If an endpoint has not been used for at least 15 minutes, i.e. it still has its full allocation of requests remaining, the reset time will always be 15 minutes from the current time.
Hope that helps.
