A good brush will last you ages if you take care of it. It should have a solid handle and a ferrule that is firmly attached, it should not rattle or feel unbalanced, if you struggle holding it properly you will end by being upset and not using it, been there many times.
The head should have a shape that applies and blends makeup smoothly and evenly, it shouldn’t been scratchy or hurt your eyes or face in the process.  The bristles should transfer makeup effectively and cause none or only very little fallout,  sometimes fallout cannot be avoided but the head should give you enough control for a neat application.