Since this didn't go quite as expected, I thought I'd share my experiences. Save others the surprises.
I started Reindex and twelve hours later, it was still working on my TM folder (which includes two million-segment TMs). At least it looked like it was still working. The only indication was that there was no "Not responding" message on the title bar and that Task Manager showed the computer was working.
I needed to get to work, so I elected to click the Cancel button. It took about an hour for the cancellation process to complete, likely because I cancelled while it was working on one of my huge TMs.