I have made it 5 days dry a few times (5.2 at the very most) before thirst got the better of me and had to work up to that starting with a day or so and gradually building upon this, but everybody is different.
My most recent summer dry fast (about a week ago) went only 3 days before i was too thirsty to comfortably continue. Your water-only fast preceeding it should prepare the way nicely by eliminating a good part of the free floating (less embedded) toxins which make us thirstier. I went about 17 hours dry and drank upon fairly strong thirst before then being able to make it the 3 days. It may have been tougher partially because my prep was far from excellent in this particular instance, leading strait in from a large and complex meal.