You can make online payments to the IRS as often as you like. Generally, it's every quarter. See How do I make estimated tax payments?
No, it's not automatically 15%. Generally, after each quarter, you can estimate your taxable income and send in the tax amount. See Can TurboTax calculate next year's federal estimated taxes?
Note: if you live in a state that has an income tax rate, you may also have to pay estimated income taxes.
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