If you're doing a true back door Roth it means you are beyond the income threshold for receiving a tax deduction on your traditional IRA and you're also phased out for income purposes regarding a Roth IRA. This means the money going into the IRA should be after tax dollars which will then be transferred into a Roth. You will receive a 1099R to be used for filing on your next tax return where no additional taxes should be paid.
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You don't pay tax on a "back door" Roth IRA conversion.
If you are doing a regular Roth IRA conversion, the taxes are due in the quarter the income was realized.