It depends on the offer. Some offers end at a specific time on a specific day and others end after a certain number of people purcahse the offer.
Purchases happen through the app using PayPal which allows you to pay with any credit card, debit card or bank account.
When you purchase an offer the items you purchased will be in the app in the 'My Stuff' area. Simply locate the items you purchased and when you are at the merchant, present the items using your phone so the merchant can verify your purchase.
The official rules can always be found at http://www.beerbybikebrigade.org/sweepstakes.
Almost all BBBB Sweepstakes have limits on the number of times any one person can enter whether they enter for free or give a donation by purchasing an offer through the app. The limits and official rules for each Sweepstakes can be found at http://www.beerbybikebrigade.org/sweepstakes.
Drawings are held when a minimum amount of donations/purchases have been met. If you enter a Sweepstakes you will be notified by e-mail and through the mobile app prior to the drawing so you are aware of the schedule. The drawing details and official rules for each Sweepstakes can be found at http://www.beerbybikebrigade.org/sweepstakes.