ZOS give an update of new stuff every month. Seems that there is a cadence of a month for release of new stuff of similar items (ie. costumes will release around the same time every month, pets around the same time every month) but there will be multiple items released in the store at a cadence of two weeks apart
Kalthar Wolf-Brother – EP Templar - 50 Maeli Valen - EP NB - 50Naps-During-Trials – EP Templar - 50Rulnakh - EP Sorc - 50Azrael Krieg - EP NB – 50Uvithasa Telvanni – EP DK – 50More-Tail - EP Warden - 50Narile Galen - EP Sorc - 50Bone Soldier - EP Necro - 50Naps-During-Trails - EP Necro - 50
So is there a standard timeframe/interval to the rate at which costumes are being added
Not exactly standard. But you can get next month's schedule in the last couple days of the month. The Crown Store Showcase on the site will list the new offerings for the next month.
New stuff is added every month, and most months have a couple limited time items that are only available for a few days.