If it is a 40+ minute game and you have under 200cs, ADC might not be for you.

Unless new to the role, don't bother playing as the ADC if you usually average under 5 cs/minute. You will just be hurting the rest of your team. ADC is one of the larger farming roles along with mid. If you cant consistently get last hits to a decent amount (you don't need to be 10cs/minute but should be more than able to get a CS count of at least 5 per minute if you want to be the ADC/Mid, at least in most of your games). If you just can't, then switch to a less farm intensive role like Jungle or Support.