Merger & Acquisition deals could infamously get dragged over time, sometimes for valid reasons and sometimes for reasons out of anyone's control (due diligence, unexpected business risks, employee turnover within M&A teams etc). Among number of goals to be achieved through a M&A deal, the main one is profitability and as the deals get delayed, company's overall profitability goals change and inevitably brings you back to reassess the synergy assumptions and re-align with the changes in the profitability objectives. So all to say close the deal as soon a proper M&A process allows, and keep in mind, synergy assumptions and lead time to close a deal tend to dance together.