You can certainly prolong the freshness if you store your ingredients in the fridge. It also depends on what type of ingredients you're using. If it's an extract kit, DME will last a lot longer than LME with everything else being equal. But with LME you can put it in the fridge, and some people even freeze it I've heard. When it comes to yeast, dry will also last a long time whereas liquid does have a bit of a shelf life. As long as your hops are in a vacuum sealed package, you can throw them in the freezer for a long time without much concern.
Where you'll start to notice a difference in quality is if you order pre-crushed specialty grains or grains to do a partial mash. Once the grains are crushed you want to use them as soon as you can, although storing them in an airtight container in the fridge will help them last longer.
So, look at the type of ingredients you intend on using and then decide whether you will be able to store them properly. But if you're brewing just once a month and will be dealing with liquid yeast and a lot of already crushed grains, I probably wouldn't buy more than two kits at a time which would put one kit at 2 months old by the time you get around to brewing it.