The "simple" answer is that its probably the created_by / created_date on the CMN_SEC_ASSGND_RIGHT table.
Why this gets "complicated" is that its quite complex how the rights relate to objects / users / groups and so on.
Also if the rights are gained by group membership, then its often more interesting to look at when (and by whom) the user was added to the group (so table CMN_SEC_USER_GROUPS) since the CMN_SEC_ASSGND_RIGHT holds the rights assigned to the group then rather than to a specific user.