[Fiona]

This is extreme, but so is the reduction:

Get a stable and fully waterproof box that can be closed and that is large enough so that you can just about fit inside.

Also get a (stable) tube that you can fit into your mouth and that is not going to close up when exposed to outside pressure and through which you can comfortably breath.

Enter the box and have your girlfriend fill it up with wet sand and/or moist earth so that you cannot move at all. Have her fill the box entirely so that it can be closed and tape the lid shut. Before she does that however you will insert one end of the tube into your mouth and create a hole in the lid through which the tube can fit. Make sure that the tube is put through that whole so that you can breath and so that she can give you some water through it.

You will stay that way for 48 hours during which your girlfriend will occasionally give you a little bit of water (or some liquid that you don't like) as well as pre-jewed food through the tube. Since you also need that tube for breathing this means that you will have to consume everything immediately.

You should definitely wear your chasitity-belt while being burried, but a large butt-plug and a vibrator that your girlfriend can control from the outside might be nice additions as well.

You may also want to cover your hair and your eyes before entering the box and in order to make it more fun your girlfriend could also put you into bondage first.

While the pressure in the box should be as high as safely possible, make sure that you check in the beginning that you can somewhat comfortably breath.

In exchange for completing that dare AND writing an extensive report on how you felt doing it, you will get a 50 day reduction.