The snapshot is made immediately, with no wait. If you set aside 100 GB for the snapshot, then after 100GB of changes have been made to the original volume, the snapshot becomes inactive and can no longer be accessed (and any data different between the snapshot and the original volume is forever lost, but the original volume is unaffected).

Snapshots are useful for making backups of a volume. Snapshots are only used in order to make sure the data doesn't change while backing up from the snapshot to some external volume.

It's usually a good idea to assign for snapshot space about three times as much as your changes will be during the snapshot.