If you shoot a caliber that is relatively cheap now, I would stock up in large quantities now.
For example, if you like 5.45 AKs, buy at least 10-20k rounds, since it is so cheap right now, and will likely only go up in price, you can have either a lifetime of ammo, or several years worth, depending on how much you shoot. (Or a few months worth, if you are like some people lol) My way of looking at cheap surplus is, remember how cheap surplus 7.62x51 was years ago? Now it's around 50 cents a round, even if you don't shoot it, you can always sell it at a big profit.
5.56 ball is pretty cheap right now too,(at least compared to 08/09, not like back in the day when I was a kid
) once I move into a bigger place in the next few months, I'll be buying cans of that federal M855 on strippers fairly regularly. This is in addition to regular purchases of bulk food, medical supplies, and some precious metals. I'll stop buying ammo, and other stuff when I run out of room, or if the price goes up to a point where I feel it's not worth it.
I will say this, you are far better off with good instruction in how to use your guns, and a few hundred rounds of ammo, than having a safe full of them and a pallet of ammo.