Short answer: If you do not sign up for Part B when first eligible and have no Special Enrollment Period, your premium rises 10% for each full 12 months you could have had it, usually for as long as you have Part B.
The penalty exists to encourage people to enroll on time. For example, if you went 24 months (two full years) without Part B after becoming eligible, your premium would be 20% higher. The increase is calculated on the standard Part B premium and charged on top of it every month, by federal law.
You can avoid the penalty if you qualify for a Special Enrollment Period, which generally applies when you delayed Part B because you had health coverage from your own or a spouse’s current job. The late-enrollment penalty is separate from IRMAA, the income-related surcharge that some higher earners pay on top of the standard premium.