I think the key question here is what is the size of the transactions that are being invovled here that will give a pretty good idea. Based on my personal experience, I have been part of an integration project subsequent to acquisition. In view of the complexity and the restriction of the type of industry with profound impact, the integration process where I was part of, took many years even though acquisition was completed some time way back.
With huge acquisitions, there are much more structure and communications, henceforth, all these are tracked on a weekly basis via the PMO till completion. Henceforth, it is more well defined.