Are both sentences Ok?

I'll do it when I want, not when you want.
It must be when I say, not when you want.

Question

Are both sentences Ok?

 I'll do it when I want, not when you want.
 It must be when I say, not when you want.

Answer

Yes, they are both OK, although the first one sounds more natural.

Answer

I´ll do it when I want, not when you want.

Alternatives:

I´ll do it when it suits me,not when it suits you.

I´ll do it when it´s convenient for me , not when it´s convenient for you.

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Good luck on your learning journey!  : )