Within 2 hours of grinding, you will notice a loss of quality. Grind immediately before you brew . Achieve the finest grind possible without clogging the paper filter, or, if using a presspot, releasing too much sediment into the cup. I find that people do not usually grind fine enough. The purpose of grinding is to effectively increase the surface area of the coffee exposed to the water so as much of the flavor oils can enter the brew.
Fine grinds result in more flavorful, economical coffee.
If it's so fine that it produces undesirable sediment in the cup, or the brewing time is lengthened because the paper filter clogs, you have gone too far. Use a good grinder. The whirley blade smashes the bean to bits by rotating at high speeds. Powder clogs the filter, and boulders don't release all their potential contents into the brew. It is also difficult to reproduce the same results with the whirley blade.