The answer is "It depends ..."
If it's the only VM on the datastore for which you are going to create a snapshot, and none of the other VM's have thin provisioned virtual disks, then the required free disk space equals the amount changes in the VM's guest OS (disk write activity) for the time that the snapshot exists, plus some additional free disk space that may be required for log files etc., and some space for deleting the snapshot again.
For e.g. DB, Exchange, or file server this could be quite some disk space. However, nobody can give you an exact number. I'd suggest you start with not less than ~20% of the VM's virtual disk's provisioned size, and then monitor disk usage.
André
