Something like 10-20% a month should be fine. There are a lot of problems that water changes can prevent:
http://reefkeeping.com/issues/2005-10/rhf/index.php
The DIY 2-part all add NaCl (table salt) over time because the primary ingredients are calcium chloride and sodium bicarbonate. Water changes and dilution are the key to keeping the ionic balance and salinity in the reasonable zone.