The least possible tax is zero, if it was a Roth IRA and you are over age 59-1/2. The most possible tax is 60% (!) if this is a pre-tax IRA or 401k, you are under age 59-1/2, you live in a high tax state like California (13% tax rate), and you have other taxable income.
It depends on your age, filing status, other income, what state you live in, and type of retirement account. You would have to use Turbotax or an estimator like the TaxCaster to get an estimate that fits your facts.
TaxCaster tax calculator
