In my experience this is all dependent on the complexity of the integration, thus putting a specific time frame too it is very difficult. But, most of the more complex integration I have done ran between 1-3 years and the biggest part that pushed out the timelines was the integration of the different technologies. Multiple regions and jurisdiction can also influence this as there are regulatory, compliance and financial requirements that can increase the time to complete the integration.
We aim for 120 days from Day 1. Now that does not mean the integration is complete, but it does mean that the key elements of the post-merger integration has been assigned to the proper teams, and the post-merger integration elements are now really “business as usual”.
Key areas of culture, communication, employee benefits & payroll, training, etc. area all taken care of. The integration is now in a specialized state.
For example, as a technology company we have a lot of R&D in our acquisitions. Within 120 days, we have aligned all R&D people to our spectrum of talent, and they have been trained on expectations and growth paths in our company. There will still be a lot of R&D integration work to do, but the primary people integration elements are complete.
The complete integration would appear to be 2-4 years for us, depending on the size of the acquistion. We are only in our 3rd of integration and have performed 4 acquisitions in that period of time.