You start to lose a suntan when the uppermost of your skin cells are shed through the natural skin renewal process.
This usually happens within a week or two of getting the suntan.
So some peoples suntans will go away within 1 week while other peoples sun tans may last up to 2 weeks before going away.
In 28 days your skin undergoes a full renewal process so you'll certainly lose the suntan before those 28 days or within those 28 days but usually you'll lose the suntan within a few weeks of getting the suntan.