Freshly milled flour is at peak freshness the day it is milled, but you can keep it at room temperature for up to three days. You can store it in the fridge for up to a week, and in the freezer for up to six months. That said, it will likely be fine at all of these temperatures for longer periods of time--it's not like it will make you sick or anything like that. But after the flour is milled and exposed to air, it starts to oxidize which begins the degradation process. Most people milling their own flour are doing it to get all the benefits, including peak nutrition, so it makes the most sense to use it soon after milling or store it in the freezer.