Most law departments have available the data they need to create benchmarks going back three to five years. As an example, the law department can count its total headcount on December 31st of each year and use the revenue for that year to calculate legal staff per billion of revenue. All of the common benchmarks permit retroactive calculation.
If a law department does not want to spend money for a survey or conduct a bespoke benchmark project, it can still create its own set of historical data, what I call time series benchmarks, and see whether trends point in the right direction. {See my post on July 25, 2005 about embedding numeric goals and April 9, 2005 comparing law to other staff functions, and Aug. 3, 2005 on patent benchmarks.)