This would be better than the above...

In fact, it would not waste much processor time at all to do the simple math at the end of every bucket to update a running "Average Time per bucket" and a "Estimated Time to Complete" should be quite easy

If it started at 12 noon, (100 buckets) and 1st bucket took 5 minutes, second took 10, third took 15, then at end of third/start of fourth, they could update display:

Render Started: 1200
Buckets Remaining: 97
Avg Time/Bucket: (30/3) = 10min
Estimated Time left: (97x10min) =>16.16 hours

Of course, it should not give an estimate until it gets started (i.e. 10 buckets or so), similar to how windows does when copying files.