Troubleshooting PHP timeouts
A PHP timeout occurs when a Drupal or PHP request takes longer than five minutes to produce output, terminates, and produces an HTTP 500 error. Most web browsers, depending on the client, don't wait longer than two minutes before discontinuing a request and displaying an error.
Long-running processes consume server resources. If there are too many processes running, it impairs the performance of your website. The Acquia Cloud web stack accommodates requests that require extra time to complete, but does not to allow a process to run indefinitely.