In order for a grandparent to file for legal guardianship and it be granted how bad/unfit do the parents have to be?

Hello:
As a nonparent, you do not have STANDING to seek custody rights over your grandchild in family court unless you first file a guardianship petition in probate court. If the child's father objects and is also seeking [sole] custody rights over the child, then the next step would be to consolidate both the guardianship proceeding with the family court custody proceeding. Then it would be up to the court to decide which custody or guardianship orders would be in the child's best interests.

Based on your description, i.e. that you have been living with your grandchild for a significant period of time and the child's mother may not be interested in assuming parental/custody rights, it appears that a guardianship petition would be appropriate. The reasons which you have cited, i.e. parent's mental/emotional instability, domestic violence and substance abuse, are common legal grounds for seeking guardianship. Since a guardianship order does not legally terminate the parental rights of the child's natural parents, the court will not focus so much on whether they are parentally unfit as on whether the guardianship is necessary to prevent detriment to the child.

At a later time, the court may restore parental custody to the child's father [and mother] when the circumstances have changed.

[Note: The above information is for consumer education purpose only, not intended to be legal advice to be relied upon, and does not create an attorney-client relationship]