What version of LVDSC are you using?
There may have been a 'bug' in LVDSC v6.0/6.0.1. I had some correspondence with NI support in 2001 that indicated a Citadel database would only pay attention to the value of "Days to keep historical data" that it was inititiated with. Subsequent changes were ignored.
It should be noted that in using "Days to keep historical data", data is only marked as old and will be overwritten in time. In other words, the old data does not disappear immediately.
I have a dim memory of doing a quick test with "Days to keep historical data" in LVDSC v6.1 and having it work as desired, but in v6.1 Citadel DB management VIs appeared and I switched to using those to control database size. Archive Database.vi has the archive option 'destructive', which deletes data from the original database immediately.
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Fading out. " ... J. Arthur Rank on gong."