You can create persistent disk snapshots at any time, but you can create snapshots more quickly and with greater reliability if you use the document [1] as best practices.
For most situations, you can use the Volume Shadow Copy Service (VSS) to take snapshots of persistent disks that are attached to Windows instances. You can create VSS snapshots without having to stop the instance or detach the persistent disk [2].
VSS snapshots are intended for backup and recovery purposes on systems with VSS-aware apps. Use VSS snapshots only for backing up data from persistent disks. Creating VM instance boot disks from snapshots of running instances is not reliable.
[1] https://cloud.google.com/compute/docs/disks/snapshot-best-practices
[2] https://cloud.google.com/compute/docs/instances/windows/creating-windows-persistent-disk-snapshot#create-snapshot