How much time do you spend evaluating vendors each week? E.g. Reading up, having phone calls and demos etc. 

This is highly variable. When starting a new project to solve a problem it could be 10-20 hours a week. In a budget tightening cycle it will be zero hours per week for months on end….

I believe in PoC with real sample data to test the outcome for selected products from shortlist,

Most of my time is spent reading to gain an understanding of the tool capabilities. If the a demo is available, I try to navigate the tool to experience how easy it is to use and the reporting capabilities.