The answer is… NEVER!
Sharing your credentials is an ARMLS violation. Per Rule 7.2 subscribers may not share their access credentials with anyone, whether the other party is another ARMLS subscriber or a non-ARMLS subscriber.
If you’re ever using a third-party company to assist you with pictures or a virtual tour and they ask for your MLS log in, stop right there! Do not give out your MLS username or password. If it is a legitimate company, they will have gone through the proper steps and channels to get their own admin credentials and will not need to ask you for yours. An affiliate, such as a home or termite inspector, will need to go through an association to become an affiliate. This grants them key access but not MLS access.
If you’ve ever shared your credentials with anyone, make sure to change your password right away.
To learn more about admin access, read up on the New Admin Policy in the MLS.