Does that usually happen every couple of weeks (i.e. do you re-do a braid after 2 weeks, etc.)?
I can't really tell how soon it happens. I wash my hair twice a week, and human hair itself slides when you wash. Maybe not the first wash but maybe the third. And because I'm always feeling through my hair I might find one that needs redoing today and a few others and only do some. Then I'll get to the ones I didn't do tomorrow, but there might be a few new ones. So I'm always redoing one or another. And because they are so many, I really couldn't tell which one was done when and how long it was till it needed doing again. Also, by the time I get to the last one, the first one probably needs redoing. And since I have HIH, this is all very exciting to me.
When it comes to my hair, I rarely do a lot of thinking/calculating, which is probably why I enjoy my hair so much. Not having times or dates control my regimen really does make the journey less stressful
Another thing to consider is growth rate. If your hair grows at the normal rate (1/2 inch per month), then you'll have that much growth anyway in a month; so that's the least amount of hair you should have below the starting point of your braids. Then human hair might slide when you wash, especially if the braided part isn't firmly done, so again the braids may get a little farther away from your scalp. Which is why some people will see a lot of "growth" while wearing braids and then find when they undo their hair, it's not as long as they thought.
So, really, rather than wonder how long it takes for me or anyone else before they need to redo their hair--because after all, everyone's growth rate is different so that info won't be helpful to you--just make a point to get your braids redone when they get loose. At least make sure the perimeter of your hair is always neat and firmly braided to avoid problems.