Theres a variable called $post[edit_time] so you could do something like
HTML Code:
<if condition="is_member_of($vbulletin->userinfo, X,X,X)">
<if condition="$post[edit_date] !< 12">
Edit post link
</if>
</if>
I highly doubt that will work because $post[edit_date] will not be in the format I posted, you will have to look into it more. But that is the general jist there...I hope Ive helped a bit even if it is a bit rudimentary