Note that, in general, deleting spooled files isn’t what recovers the space. Space is recovered by running RCLSPLSTG (Reclaim Spool Storage). Also, you didn’t say anything about your environment such as OS version/release. Also, you only showed output from a TAATOOL command with no info from system commands for comparison.
If this is DLTOLDSPLF from TAATOOL and “Display Spool Size — TAA” is also a TAATOOL command, then only TAATOOL support can answer with much certainty.
However, if you’re intending to cut the retention period from 120 days to 90 days, I’d guess that you could perhaps recover around 27GB… assuming you reclaimed spool storage after spooled files were deleted.
Personally, I doubt that I’d be allowing spooled files to stay around much more than 30 days. The purpose of spooling a report is to print it. A spooled file is not for long-term storage — that’s what the paper is for (or the .PDF or whatever).
You might investigate the EXPDATE() and DAYS() attributes of spooled files in V5R4 and later. And also look at Delete Expired Spooled Files (DLTEXPSPLF) in V5R4.