An important question that is as relevant to Q&A as to systematic reviews. This Annals of Internal Medicine article (click here) attempts to answer this question. The important bit, for me was this:
"This survival analysis of 100 meta-analyses indexed in ACP Journal Club from 1995 to 2005 found new evidence that substantively changed conclusions about the effectiveness or harms of therapies occurred frequently within relatively short time periods. The median survival time without substantive new evidence for the meta-analyses was 5.5 years. Significant new evidence was already available for 7% of the reviews at the time of publication and became available for 23% within 2 years."