The exact length of the Pokémon Trainer Club downtime is unclear, with different sources stating different time lengths. The announcement notice on the official Pokémon Community Forums, plus the announcement post in the Pokémon UNITE News & Announcements board, both state there will be an estimated 1 hour downtime, however the announcement post in the Pokémon TCG Live board states the downtime will be 2 hours long. All three of these sources also erroneously state that the downtime will start at "9:00 AM PDT (13:00 UTC)", despite that PDT is already over, and that it wouldn't be mapping to 1:00pm UTC even if it wasn't. The notice in the Pokémon TCG Live app, which correctly notes 9:00am PT corresponds to 5:00pm UTC, also gives the downtime length as 2 hours.
After the maintenance is completed, players will be required to re-consent to allow the app to access the information in their Pokémon Trainer Club account the next time they log into each app with their Pokémon Trainer Club account. The actual permissions being granted will remain identical to what they were previously, and players will have the opportunity to review these in-app prior to accepting or refusing them.
Pokémon GO has specifically advised players to consider attaching additional login methods to their Pokémon GO account, in order to ensure that they do not lose access should there be any issues with the service moving forward. Pokémon GO will also be offering players an in-game reward for any players linking their Pokémon Trainer Club account to Pokémon GO for the first time.