This is synthetic (man-made) m-RNA. Will not degrade or be inactivated. Seems like it varies from person to person. They have started doing autopsies on those dying after vaxx and finding spike protein in their tissues. There is a way to distinguish between viral spike protein and vaxx spike protein. I am sure we are going to find out details of this real soon.
This article indicate spike protein in lymph nodes for two months.
This one says spike protein in circulating exosomes after four months.
https://www.jimmunol.org/content/early/2021/10/11/jimmunol.2100637