How long is too long to wait for getting a response to an HTTP request?

Let's say I am creating a service which can take a request from a user, do a calculation based on that request, and then get back to that user with the result of that calculation. How long is too long where I should consider not doing this process in one HTTP request? What is best practice around long-running calculations triggered by an HTTP request? Is there some benchmark for how long is too long for a single HTTP request to respond in general?