Leave it for a minimum of two weeks.
IMO there's two main things to consider with FG.
Time - Depending on the ambient temperature, beer can reach FG either quickly or slowly. i.e. higher temperature = quicker fermentation; lower temperature = slower fermentation. Consistency of the aforementioned temperatures also plays a huge part.
Mash temperature (non-applicable to kits) - If you mash on the higher end of the temperature range, then your wort will contain less fermentable carbohydrates. What this means is that your FG will be slightly higher. i.e. 1.016 vs 1.009 for example. They're random numbers by the way.
The textbook approach for determining when your beer has reached FG is three consecutive gravity readings over three consecutive days. Assuming fermentation temps are consistent and correct, etc. As I'm lazy, I just leave all my brews for 2-3 weeks anyway. I have a good idea what my FG is going to be anyhow as temperatures are well controlled. So I just measure once and if the number is right, it's done.