Getting a rubber powered plane to fly as long as possible depends both on the total weight of rubber you use and the total width of the rubber strands you use in the motor.  A plane weighing 1/2 0unce might have a good flight powered by 1 gram of rubber, arranged in 2 strands, each 1/8 inch wide..  The prop for such a plane might be about 5 inch diameter. 

A  scale plane weighing 3 ounces might have a motor weighing about 10 grams, arranged as 6 strands of 1/8 inch wide rubber. The prop for such a plane might be 8 or 9 inches in Diameter. In either case, when you wind, you pull on the motor to stretch it. as you put in the turns with a geared winder, resembling a hand drill.