>>I thought percent_complete in the dm_exec_requests table would be the percentage you seek.
>
>Well, at first, I didn't see that field. So, I ran the test again. This time, I could see that there is a Percent_Complete field. However, this one remains at 0 for all records, despite the fact that I have a big index in progress for 5 minutes.

The percent complete field did not change on subsequent queries of the dm_exec_requests table while the index ran for the 5 minutes? The docs in the link I sent you say it should update for ALTER INDEX REBUILD.