I would think the general/standard recommendation is to break with at least half the time you cleansed, so 1.5 days. However, I did a 3-day and broke over a half day. I must add though, I am used to going up to 2 days at a time without eating much or at all, so it was no real shock to my system.
How comfortable are you with your body/system? Do you know your body/system well? I ask because so many people are so busy that they don't have (or take) the time to listen to their body. Either way, simply take it easy and listen to your body =)