My article just came out in the National Law Journal, June 12, 2009, on incumbent law firms. I defined them as firms that (i) have represented you for several years (ii) in multiple matters (iii) for significant fees. I think five years of representation is a minimum to be anointed an incumbent. The matters need to cover a significant range of work in an area of law, as measured against the total number and fees in that area. As to the significance of fees, they ought to be material to the client year after year and there will likely be a relationship partner at the firm (See my post of July 26, 2008: relationship partners with 8 references.).
Those three factors define incumbency: extended time of use, large range of services, and major amounts of money. Your incumbents are your primary firms. Most legal departments larger than five lawyers but less than twenty lawyers have teamed with only a handful of incumbent firms. Over a period of two-to-four years, incumbents are likely collectively to account for well more than half of the total fees paid by the department.