After you register a VM to a host pool within the Azure Virtual Desktop service, the agent regularly refreshes the VM's token whenever the VM is active. The certificate for the registration token is valid for 90 days. Because of this 90-day limit, we recommend VMs to be online for 20 minutes every 90 days so that the machine can refresh its tokens and update the agent and side-by-side stack components. Turning on your VM within this time limit prevents its registration token from expiring or becoming invalid. If you started your VM after 90 days and are experiencing registration issues, follow the instructions in the Azure Virtual Desktop agent troubleshooting guide to remove the VM from the host pool, reinstall the agent, and reregister it to the pool.