A lawyer I know, responsible for mergers and acquisitions at a growing financial services company, offered some comments that deserve passing on.
This lawyer has concluded that international M&A deals, compared to domestic US deals, take three times as much time and five times as much money. The linguistic, cultural, and legal complexities of acquisitions outside the US pump up both hours and dollars.
His second rule of thumb holds that a billion-dollar deal requires a good in-house lawyer full time. Strikes me as a plausible benchmark for staffing. Not that the benchmark is linear: a two-billion-dollar deal does not need two FTE lawyers. Look at the size of deals, consummated and not, that your company undertook over the past few years, and test these experienced estimates.