How long can it take muscle to waste? my dominant arm feels weak but i can still lift things. it also looks skinnier.

Need reason: Muscles waste, or atrophy when they are not used, (use it or lose it) because of being in a cast, bed bound or a neurological problem. It would not be normal to see wasting in an arm that is being used normally. After a week of no use you can see it. If there is no obvious reason for this then see a neurologist. Good health.

Weeks to months: If you have muscle atrophy you need to see your doctor for an evaluation.