When do I pay taxes on a Roth IRA conversion if it’s done in July 2020?

[dmertz]

A Roth conversion done in 2020 is reportable on your 2020 tax return.

The US tax system is a pay-as-you-go system.  See IRS Tax Topic 306 regarding the safe harbors for having sufficient taxes withheld or estimated taxes paid to avoid an underpayment penalty for not paying the taxes timely:  https://www.irs.gov/taxtopics/tc306

The income is realized on the date of the distribution from the traditional IRA, so a distribution from the traditional IRA in July is 3rd-quarter income.  The due date of an estimated tax payment for the 3rd quarter is September 15.