It takes 3 days for a failed PayPal payment to become void. PayPal payments generally fail because your order total changed during proofing. If your order increases by more than 20% then PayPal will fail when we attempt to charge you upon proof approval. If you decide to change to paying via credit card then the PayPal payment will remain pending for 3 days before it expires.
We plan to eventually improve our integration with PayPal to void the payment automatically if you switch methods. For now, if you need a payment voided immediately contact us and we will manually void it for you.