The purpose of estimating TAM is to apply an early litmus test to see if it will be worth your time to work on this business as it is currently conceived. And if you are raising money, it's a litmus test for your potential investors to see if they should spend time with you. So spend only enough time to get that gut check, but don't get carried away. There is a company to build and it needs your time more than an elaborate spreadsheet.
This article builds on content developed by the Martin Trust Center for MIT Entrepreneurship for MIT's Orbit Knowledgebase and is licensed under CC BY-NC-SA 4.0.