Honestly, I'd say "as needed" but I try jump in when the timing is best in terms of bang per buck. In the 3D era I've owned (in order): ATI Xpert@Play, Voodoo II, Radeon 8500, Radeon 9800 Pro, Radeon X850XT, Radeon HD4850 and I've currently got a Radeon HD 7950. I'm waiting to see what Vega does to pricing and whatnot but I'll likely upgrade this year. So that means a new (or in the case of the Voodoo additional) card in 1997, 1998, 2001, 2003, 2005, 2008, 2012 and likely 2017. So like what, every 3 years or so with a recent outlier caused by the loss of the 20 nm generation parts?