for 5 months. That means if a person doesn't file until October 16th, the IRS will consider the filing 7 months late. That portion of the penalty would be 25%. If they didn't pay, then there is a failure to pay penalty of 25%. That is a total penalty of 50%. Luckily those penalties max out at 5 months for a total penalty of 50%. However, interest goes from day one until paid.