some say 3 months some say 6 months. i say send a message to the last group uploader and see if they dropped... unless its LH than id say 3 months
here are MD rules on it
group that is locked, has missing chapters, and has had no new releases on MangaDex or any websites affiliated with that group for a period of 6 months is eligible by the request of any user to become marked as inactive.
Upon receiving an inactivation request, MangaDex staff will attempt to contact and notify the group.
It is possible for the notified group to request becoming exempt from being marked as inactive [for a period of up to 2 months].
If the group is not exempted, it will be marked as inactive after 2 weeks from the initial notification.
If a group is not exempted, they may still request for the chapters of a certain series to be removed (such as if an existing official licensed version exists).
An inactive group will have all of its upload restrictions lifted and its leader and members will no longer have automatic editing and deleting permissions to the chapters associated with the group.
Should the group resume releasing new content, it will no longer be considered inactive and the normal permissions will be restored.
Chapters from groups linking to group sites which adopt excessive profiteering techniques are not allowed. Examples include (but are not limited to) adblocker-blockers, pop-ups and pop-unders.
Uploading a scanlated chapter before the official raw chapter is released in the original country is prohibited, unless an appropriate delay is used.