It depends a lot on the humidity and heat or light in the environment where the microfiche are being stored. But they should be able to retain their data for 500 or so years.

CDs and Laserdiscs are also seeing bitrot. The layer of material that is etched does degrade over time. Error correction helps some, but if it's a writable CD or DVD it's only likely to last a decade or two. M-Drives are CDs that are designed to retain their data for about 1000 years and can be writable by specific consumer drives. Not sure how long the professionally pressed CDs last but it's not that long.