The EPA issues Health Advisories for chemicals that includes guidelines which offer an estimate of acceptable limits for daily consumption that are not expected to cause adverse health effects to vulnerable populations (such as infants, pregnant women, or elderly persons).  A ten-day health advisory refers to a concentration that is not expected to cause adverse health effects for up to ten days of consistent daily exposure at that level.  This is based on a 22 pound child consuming one liter of water per day.
A Lifetime Health Advisory refers to a concentration that is not expected to cause adverse health effects over a lifetime of consistent daily exposure at that level.  This is based on a 154 pound adult consuming two liters of water each day.  These advisories are not enforceable standards, but are meant to serve as guidance and are based on scientific studies.