It depends on the water you are treating.  For water in disasters and emergencies the World Health Organisation recommends 5ppm.  If the water is visibly dirty or cloudy it should be filtered with a cloth to remove the sediment.

For clearer water or for Household Water Treatment the Centers for Disease Control and Prevention (CDC) recommend a dosage of 2ppm.  In this case Oasis Home is the tablet you require.  This sub brand ensures there is no confusion between these two different situations and dosages.  

Oasis Water Purification Tablets have a shelf life of 5 years.

Yes.  If there is a lot of sediment or matter present in the water then filtering through a cloth or filter is recommended before treatment.

Water should be left for 30 minutes before drinking