You should always aim to reply within 24 hours. If you are unable to, for example you are going on holiday. In this instance you should contact your Unibuddy admin who can temporarily deactivate your profile.
If you miss the 24-hour deadline, try and respond to the prospective student as soon as you can - being sure to apologise for the delay. Your admin will be able to see when you have missed the 24-hour deadline and may intervene to let the prospective student know you are not available.