Gang,
I have an inventory that started out on with big chief tablet and no. 2 pencil about 35 years ago. Later, I converted it to IBM punch cards. Still later, I converted it to a PC based spreadsheet. Since then I have made numerous changes to the format to accommodate changing tool interests and research interests. At present, I have two inventories, one for modern working tools and one for vintage collector tools. I run off updated paper copies of both inventories about 4 times a year and keep them in a 3 ring binder. I take the binder with me whenever I'm out looking for tools; it saves a lot of needless duplicate purchases.
I have found that it takes about 30 minutes a week to keep the two inventories up to date; it takes much longer to research the tools (date range, OEM, replacement value, etc) than it does to enter them in the inventory.
For insurance purposes only, I also do a quicky video inventory at the end of each year. This takes about an hour and is really easy to do.
That said, I would be hesitant to build an inventory from scratch today. It would be overwelming. There have been guys who have done this, they took it drawer by drawer (i.e. drivers, wrenches, pliers, sockets, etc), and gradually built up their inventory as time allowed. Still, I wonder how they came up with purchase dates and purchase prices. Perhaps they just estimated these items. YMMV