You’re guaranteeing the full amount for the length of the agreement the tenant is signing for. Tenants often sign up for six to 12 months on a new agreement. After this time there will be rolling notice period, which can vary.
As a guarantor, you have full responsibility to pay what’s owed. If even just one payment is missed, the creditor can contact you to demand you pay what’s owed or take the money from your bank account - if you’ve signed a Continuous Payment Authority as part of the agreement.