I rarely buy more than an eighth, unless I'm going back home for vacation, where weed is much harder to come by, and then I stock up. However, since a close friend of mine got a club card recently, I've started buying 1-2 grams of a number of different strains, so I can choose the flavor of high I get. In my opinion, its better to spend money on quality (and be able to buy the same strain repeatedly if desired) than to smoke the same damn thing for months.
However, since my lungs aren't the best, I've started applying that rule to hash, so that now I have a small collection of hash & kief from individual strains. The less I have to smoke, the better I like it.