I don't think the answer is clear, but the present KDOQI recommendations are that access placement should occur once the GFR drops to less than 25 mL/min, if the serum creatinine goes greater than 4 mg/dL, or within 1 year of an anticipated need for dialysis. It also states that a new primary fistula should be allowed to mature for a minimum of 1 month, and ideally for 3-4 months prior to cannulation (AV grafts, by comparison, require a maturation time of only between 3-6 weeks according to these guidelines).
In this 2004 JASN article entitled, "Late creation of dialysis access for hemodialysis and increased risk of sepis" by Oliver et al, investigators from Canada performed a retrospective analysis of patients starting dialysis who had their access placed either "early" (defined as being placed greater than 4 months before initiating HD), "just prior" (defined as being placed between 1-4 months before initiating HD), and "late" (defined as being placed 1 month before initiating HD). Of note, the majority (3687 of 5924) were "late" access placements. Early access creation was associated with a relative risk of sepsis of 0.57 and a relative risk of mortality of 0.76, both of which were predominantly explained by an increase in catheter use observed in the "late" group. Granted, this is not a prospective trial, but it does seem to provide some rational evidence in support of early AV fistula placement.
My personal (and still limited) experience in this area: I think that many nephrologists know that early AV fistula placement is important, and this is discussed with patients at routine CKD office visits. The challenges come from two main areas: (1) actually convincing the patient to go ahead with it (there are often very powerful mechanisms of denial involved in a patient actually accepting the fact that they are eventually headed for dialysis), and (2) administrative delays in setting up a patient to be seen by a vascular surgeon. If anybody has any tips on ways to increase early AV fistula placement, please share them!