Check out this answer provided by AWS :
When the specified timeout is reached, AWS Lambda terminates execution of your Lambda function. As a best practice, you should set the timeout value based on your expected execution time to prevent your function from running longer than intended.
To be simple, after the 15 mins the function automatically stops. When requests come in faster than your function can scale, or when your function is at maximum concurrency, additional requests fail with a throttling error (429 status code).
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For more information about AWS Lambda, check out this Complete Tutorial on AWS Lambda.