When you are properly following the procedures on how to shock a pool - for example in the green algae situation shown below - if you are testing and dosing properly you should see visible improvement each day. Your results may vary but the time difference between these two pics is about 25 hours. If you aren't seeing a visible difference daily then you should review your testing procedures/results or your dosing calculations.
This is a non-swg vinyl inground pool with a sand filter. Almost crystal today ands its been 48 hours.