Maybe a week would be good. I've used it for maybe 4 nights and it started creeping but had a few more nights in it. Of course, it depends on winds and stuff.
Take a stapler for paper and put a few staples through it and it will last longer, at least that's what I found. If the staples are through two layers of duct tape and plastic, it should hold up.
I love to see creative, low cost solutions.
You might want to think about doing it in silnylon some day...