What amount of upper body exercise should i do?

Most of my exercise is based around upper body. At the moment im not quite old enough for the weights at my gym.
Because of this i do free weights and press ups/sit ups.

I've heard that your muscles build better if you rest up for a few days. That they bulk during the time off.

Is this true and can someone propose a basic schedule with regards to exercise/rest ratio?

Thanks

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