If a legal department faces less than $500,000 of spending for the foreseeable future, covering a cluster of related legal work, it doesn’t make sense to undergo a competitive bid among law firms to handle that work. It takes time and effort to consummate a competitive bid. The outcome may well be traumatic for several law firms, upsetting long-standing relations. For the reason that any change brings risk, unknowns enter the process and make it less desirable. One can always be second-guessed for deviating from the status quo.
For all these reasons, set against the likelihood that your savings would be on the order of 10 to 15 percent of the fees, I propose the approximate threshold stated above.