There is currently not a way to expand the character count on photo captions. The limit is there to make sure that photos in the gallery always look good in mobile. What is your use case for more than 150 characters? I would love to pass the request (and having the 'why' always helps us) to the product team. If you need longer text areas, maybe consider using one of the "About" or "Contact" layouts instead of the photo gallery, so you have more space to describe what's in the photo?