aye thats not hard at all
just edit the:
\SilentHunterIII\data\Cfg\basic.cfg file
around line "61" you will something like:
[TIME]
TransferFlotilla=1
NbDaysInBase=10
NbPatrolsInFlotilla=3
NbMonthsInFlotilla=6
edit the NbDaysInBase= to any number of you want.
for example:
NbDaysInBase=10 to
NbDaysInBase=30
and you will stay a minimum of 1 month in base