Dorothy Leonard and Walter Swap explain in Deep Smarts: How to Cultivate and Transfer Enduring Business Wisdom (Harv. Bus. School 2005, pg. 49) that “most evidence suggests that it takes at least ten years of concentrated study and practice to become an expert (as opposed to merely competent).”
Assuming the Ten Year Rule holds for lawyers, and assuming three years of law school starts the clock, and further assuming a typical lawyer stays in one specialty area of law, then around seven years out of law school they can be considered an expert.
Having assumed all that, when does an insider have time to overlay substantive expertise with in-house savvy, which has to do with client relations, knowledge of the business, and political finesse within a corporate structure? The answer is often that deep legal expertise remains with outside counsel, whereas the lawyer hiring that outside counsel needs less substantive expertise and more generalized skills.