Merging several non-SAP ERPs into one single SAP ERP

Hi guys,

I'm new to SAP and I have a question purely out of curiosity. Let's say you have a manufacturing company which is spread across different countries under a divisional structure and all these divisions have their own product line (segments) and their own ERP system (AX, SAGE etc.). Each ERP could contain thousands of items with multi-level BOMs. And let's say now, you have come to a level where you need to take a strategic decision to merge all these ERPs into a single one and that would be SAP ERP. Does anyone have any insight what would be the extent of such a project in terms of timing? How long it would take to have all these ERPs successfully integrated into SAP and get to the same operational level as they were in the previous ERPs. I'm not looking for a precise answer, I'm more looking to get insights from people that have experienced such implementation. The idea is to get an overview how messy this could be 🤣

Please feel free to share any opinion you might have.