It depends on the type of travel pass or ticket on your card.

• If you have a 10-journey card, single ticket or return ticket: you can get on and off all bus and tram services without limitation for 60 minutes after scanning (this excludes Student Express Lines 178, 179) and after getting on you can complete your journey even if the time limit has expired.
  If you want to transfer after 60 minutes, buy a new ticket, or scan your card again (if you still have available journeys left), even if your bus or tram has been delayed.
• If you have a day pass: This is valid for 24 hours after scanning. A three-day pass is valid for 72 hours. You should still scan your card every time you board or change to a different bus or tram.

See also ‘How long do I have after purchase to use a ticket on an electronic card?’