You can check how much free space is left in two ways.
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When you are adding the custom content. Go to Admin → Content → Add Custom Content → Upload Content. The information is at the bottom of the page in the middle
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When you are managing the custom content.
Go to Admin → Content → Manage existing custom content. The information is on the top right
NOTE: There is a hard limit on the GB storage, so there will be no unexpected bills. If/when needed, the Admin can chat with his AM to purchase additional storage.