You dose when your current water change schedule is not sufficient to keep parameters in the range in which you choose to maintain them..
At that point you either decide to increase your water change schedule or amount or start dosing..
Then you see what the consumption rate is and dose as needed to maintain the parameters where you want..
Here is a good starting read for you on acceptable parameters..
http://www.reefkeeping.com/issues/2004-05/rhf/index.php
Note: typically one would start dosing when alkalinity is dropping more than .5-1dKH in 24 hours period or any will not stay in the 7-11dkh range between dosing..
Typically a few frags will not require you to start dosing..