Sorry, 30 is the minimum number of days.
If you go into postgres (PGAdmin3), it is possible to edit the parameter called
It turns out that this is a PARAMETER in the Postgres Param table, KEEP_HISTORY_DAYS_AMOUNT column, it can be changed. Postgres admin tool PGAdmin3.
It looks like when the value is under 30, it fails and goes back to 30.
The Synchronizer client (Server Options) throws an error about the minimum being 30, but resets it back to 30 and seems to get over it. You might also need to reset Log Level and QC10/QC11 settings.
Interestingly, after changing the value through Postgres, the value is now editable from the HP-sync client, but still only allows values 30 (days) and above.
It may be best to leave it alone sine it will not allow a number below 30 anyway.