Some campaigns may have a time limit for responding. This is typically 1-2 weeks after the final question has been sent. You will see a Campaign closed message when the campaign is no longer accepting responses.
Some campaigns however may remain open indefinitely in which case, you can answer a question any time after it has been sent. Find the email/SMS linked to the question or find a recent Joyous email and click a rating button or the link that says Go to your Joyous feed. You can scroll through your feed to find reminders about unanswered questions and can submit a reply to these questions at any time.