For many years, portrait photographers dispensed “proof” prints on unfixed photo paper. The client took them home to share them with family and friends. The client generally returned to the studio and ordered a selection of finished portraits. Those “proofs” not returned soon faded. We are talking about fading in a few days of viewing in normal room light. You can do the same.
Develop your prints as you normally do. Bathe them in stop bath or just rinse in running water. Package them in lightproof material (aluminum foil works). Each time the print is viewed, ambient room light will cause some of the undeveloped/un-fixed silver salts to self-reduce. This is the fading process. At any stage, as the image deteriorates, you can bath the paper in a fix bath. This will preserve it from further deterioration.
If it’s just the fading effect you are after, you can develop normally and then turn on the darkroom white lights and watch the image fade. Again, at any juncture, you can plop the print into the fixer.
A neat trick: Develop, stop, fix and wash as normal. Soak the finished print in part A Farmer’s reducer. This is potassium ferricyanide (not particularly toxic). This action bleaches the silver image, and the paper can be bleached until the image disappears. You can then plop this dry, seemingly plain paper in an ordinary developer, and voila! The print is seen developing before your eyes. I used this trick to demonstrate what was happening to paper and to film. An effective teaching tool, because it can be carried out in normal room light.