Welcome to SCOM authoring
In order to change configuration parameters for a monitor, you need to create an override. For the override you can specify new value for snapshot age threshold and you will have to specify a management pack where to save the override (they cannot be saved within sealed management packs). We would recommend creating a separate MP with overrides for our objects, I usually call such MP "Veeam Overrides". A separate MP will help you with reinstalling or removing sealed management packs.
With respect to this particular monitor. We are recommending to remove old snapshot because it's a well known VMware recommendation. If you have a snapshot for a very long time,this could produce issues with the VM size on the disk and performance penalties, as well as issues with snapshot consolidation. That's why we have 2 days as a threshold. But you are absolutely free to override this threshold if necessary.
Let me know if you have any other questions regarding this or any other monitors in SCOM.