There is no official official period. The system makes attempts to withdraw the subscription for about a week before it reverts your account back to Free. Note that this varies depending on your bank/payment method etc.
Even if your account does get set to Free, all your music collection will stay intact 🙂 You'll only need to re-download your offline music.
Hope this info is helpful.
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“With freedom, flowers, books, and the moon, who could not be perfectly happy?” ― Oscar Wilde