Legal holds soak up time of law departments but not much has been published about the practice in the way of benchmarks. Law Tech. News, Vol. 15, Nov. 2008 at 32, offers a few as it summarizes a survey Global 1000 Companies about legal holds. Huron Consulting partnered with the Compliance, Governance and Oversight Council to conduct the study and obtained responses from about 50 companies.
Among the findings were that 80 percent of those companies issue legal holds for every matter (disputed matter I assume), the other 20 percent say they use risk and case analysis to determine holds. I take that to mean they decide each time whether to apply a litigation hold (See my post of Aug. 27, 2008: litigation hold notices with 6 references.).