The only way to get an estimate I can think of is running the compact against a SAN snapshot on similar hardware.
The free blocks could be anywhere in the file and that plays a big role.

Here are some numbers:
Current Size: 2332950 MB
Freespace at end of file: 300 MB
Total freespace: 8957 MB
Blocks Scanned: 10850564
It took 1 hour 44 minutes

The truncation was almost instant.

There appears to be this Using Log Monitor.

I use external tools to examine logs like cconsole.log though.

Was so happy to discover d ConvertShadowDatabases^MIRROR