I honestly can't really say, but it was less than 2 weeks each of the last two times it happened. My GI tract is temperamental, so I can be up 4 or 5 lbs just from the "backup on Highway 2" and then be right back down after it gets back on track. This fools both the scale and the "belt test."
For example, I lost 9 lbs in 4 days this week (Saturday AM to Wednesday AM), which was down a net 2 lbs from where I was the previous Tuesday. So, where I was trying to correct an overfeed this week, thinking I was gaining too fast last week, it turns out I lost two lbs, and didn't gain at all. I've had the opposite also happen, where I am up 6 lbs thinking its GI weight, and then clean out and am only down 1 lb, and suddenly I am looking at a 5 lb gain in fat weight.
The best method for me to avoid all this is to just weigh myself once per week the morning after my lowest carb day, after taking the browns to the superbowl (sorry if that reference gets lost in translation - It's an american football euphemism). Unfortunately, that means I only see a data point once per week, and even then, its got a lot of error in it.
I track everything I eat using an iPhone app, so I have all the food intake data. Its just a matter of getting enough samples to know when I have it sorted out. It took me two months to sort out the numbers before I was confident I had the formula for losing weight, which then turned out great. I am just back to square one again for gaining lean mass.