How long does it usually take to see a physical change when you work out?

So I'm one of those people who say. "I should work out more." Then goes and does it for three weeks and doesn't see any physical change so I stop. I know I should just keep going, but it would help me to know.

PS. I am working to get upper body muscles.
How long does it usually take to see a physical change when you work out?
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