An idle codespace will timeout/stop after 30 minutes. A free tier codespace runs for 4 hours without timeout as long as it is active, i.e. terminal commands, writing code, or the server is processing requests. An org or enterprise codespace can run longer. You can restart the codespace and continue where you left off. So essentially, it can be used forever but your server will be down during the restart. You can share your site externally through port forwarding from the codespace.