The default storage when creating a new VM in Azure is managed disks. In my case I would like to create a failover VM that will be off all the time except when I need it if I need it. Do I pay for the managed disk for the time the VM is not running?
Yes, you will be charged for Azure Managed Disks. It does not matter if the VM or other resources are accessing the storage. You are paying for disk space and the service.