One commonly cited rule of thumb for shooting expired film is to add an extra stop of exposure for every decade since the film expired. So if you’re shooting an ISO 1600 roll of film that expired 20 years ago, you should shoot as though it were an ISO 400 roll.
Source: https://petapixel.com/2020/11/18/shooting-with-35mm-film-that-expired-20-years-ago/