In general, Facebook recommends that you run an ad for at least 4 days in order to see accurate results.
So, when constructing an ad be sure to give it a sufficient budget and at least 4 days to run without any tweaks. If you aren't seeing the results you'd like to be getting after that time period, then make a couple of changes, and set it for another 4 days.
In my real life experience with running Facebook Advertising, I have found that I don't actually start seeing results until about day 4 of running a Facebook Ad anyways. From what I can tell, Facebook doesn't truly start "Feeding" the ad to my target audience immediately. It seems to take it's time to gather information before serving it "full force" to my audience.
I'd be interested to hear your experience with this 4 Day Rule for Facebook Advertising! Run your next ad for at least 4 days, and take note of when you start seeing results, then let me know in the comments your outcome!