The mixture should be of a consistency that allows the balls to be formed fairly easily and to retain their shape. If the mixture is too dry add more banana (especially if you have used small bananas) or applesauce. If the mixture is too moist add some more rolled oats.
I'm not sure how long they can be stored in a cookie jar without spoiling -- they never last that long!
This recipe is adapted from a Morsels Minus (http://morselsminus.wordpress.com) recipe for No Sugar Vegan Trail Cookies.