Required once you expect to owe $1,000 or more after withholding and credits. Skipping them costs the underpayment penalty, which is interest, not a fine.
Section 6654 of the tax code sets the requirement, and Form 1040-ES restates it every year. Tax is due as income is earned. Withholding meets the rule for a W-2 employee. Income with no withholding, which means self-employment, rental, investment, and most retirement income, meets it with four payments on the quarterly due dates.
The penalty is what makes it feel optional. A missed payment brings no fine, no notice, and no audit. It brings the underpayment penalty, interest at the IRS underpayment rate, currently 7% a year, on each installment for the days it was late. A $4,000 installment paid nine months late costs about $210. Some people pay that to keep their cash during the year, and the IRS lets them. The penalty is the whole enforcement.
Two situations remove the requirement. If you had no tax liability last year, were a U.S. citizen or resident alien for the whole year, and that return covered a full twelve months, nothing is due this year no matter what you earn. And if your withholding and payments reach the safe harbor, 100% of last year's total tax, or 110% if last year's adjusted gross income was over $150,000, there is no penalty however large the balance due in April. Pay a quarter of that figure on each due date and the requirement is met.
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Sources: IRS, Estimated taxes· IRS Publication 505· IRS underpayment penalty