I buy one whenever I see one that I like better than what I'm currently carrying. And since I've been EDC'ing my BM710 since January '06 and nothing has come along that I like the looks of more, I haven't bought a knife in a long time.
I picked up an S30V Buck Hartsook when they first came out, and a Spyderco Centofante 4 at some point since the 710, but I think those two are it.
There have been plenty of knives that I've wanted, but in the end I concluded that I wouldn't carry them because I like the 710 better, and I don't have the patience or desire for an "EDC rotation," so I don't buy.
If Spyderco would put out a D2 Paramilitary without those ugly green handles, then I'd probably be buying a new knife.
But no, there's no schedule, and I go on a knife by knife basis.