How long is someone allowed to hold a "good deed" over your head?

Say someone does something nice for you, and you say thank you multiple times, and yet they keep bringing it up. Is the size (or monetary value) of the good deed directly proportionate to the length of time the person is able to remind you? How do you say you are tired of hearing about it without sounding ungrateful?
Keep in mind that the favor was offered freely, and unsolicited. Do you ever regret accepting a favor because you were made to look like a bad person?