Hi Alex,
Your prod storage controller snapshot for the snapmirror relationship will continue to grow. Until you get your DR FAS back online and the snapmirror replication running, the volume containing the prod storage controller snapshot will continue to increasing in size.
How long you leave it will depend on how fast the snapshot is growing, how much space you have for that volume on the prod storage controller, time required for the snapmirror relationship to update when the DR FAS is back online, when the DR FAS will be operational and the relevance of data contained in the snapshot.
Bottom line is that your circumstance will dictate your available actions.
Thank You
EL