Your allowance is a recurring expense, and must be renewed in order to keep your files. It will be automatically renewed roughly half-way through its duration, so you should open your Sia client at least once every few weeks to ensure this takes place. If the entire allowance period elapses and you do not renew your allowance, your hosts will delete your files, leaving them unrecoverable.
Getting Started with Private, Decentralized Cloud Storage
Ava Howell

the blockchina lack a mechanism to notify the user ,and the user must check the allowance timely

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