How to calculate camber for a balsa glider?
Welcome to Aviation SE, Samarth!

Flight duration depends not only on camber, but also launch speed, launch angle, mass, propeller diameter, propeller pitch, rubber quality, winding tightness, etc.  I assume that by "rubber powered glider" you mean rubber band powered for the first part of the flight, gliding thereafter.

Even if you somehow find optimal values for all the other parameters except for camber, it's still simplest -- and most trustworthy! -- to build several interchangeable wings for the glider, varying camber, and directly measuring flight duration with a stopwatch.