I think I would start with one minute. With a precommitment that I will not try another minute during the next month; and a few people to observe me whether I have not lost interest in all non-wireheading activities -- and if I do, they can vote on not allowing me to try another minute for the next year.

My intuition (which is unreliable, especially when talking about fictional stuff) is that one minute of any experience shouldn't damage my brain permanently, but one day already sounds like too much.