As mentioned before, it depends on several parameters. Once is certainly snapshotting as you already mentioned, another important one is whether the VMs are thin provisioned and the datastore is over-provisioned. The amount of free disk space also depends on disk write activity of the VMs while there are active snapshots. As a rule of thumb you should keep 15-20% free disk space for datastores with either thin provisioned virtual disks or snapshots. The percentage however may vary depending on the size of the datastore.
André