It depends on how many copies they have. Run-of-the-mill Hollywood blockbusters are generally quick. A weird foreign movie may have to ship across the country. Since I like foreign movies, it has happened to me many times.
My gripe with Netflix, if any, is that sometimes shipping is delayed for unknown reasons. They send me an email that they received the stuff, then a few days may pass before they let me know the next one is shipping. Why? And why on titles marked "available now"? I seldom get more than one batch turnover per week. Things should be faster. Finally, why send me scratched discs that hiccup 5 times during the movie?? Check before shipping, dammit.
I look forward to a Netflix variant available through cable TV. "Movies on Demand" gets expensive very fast right now.