Tentatively, my App calls the heartbeat function (on the server) every 10 minutes or so.
If you have to call such a function, though, consider what else you can do with it. (Dad always said to “combine trips when you can”.)
It’s also an opportunity to return information about the App’s heath. For example, the copy running in the browser might be out-of-date, e.g., if you’ve recently fixed a critical bug, but the end-user’s copy is older than that.