Long-term degradation of film depends on a few different factors including the type of material they were stored in, along with temperature and humidity of the storage environment.
That said, from the four types of film most people are exposed to (pun intended), B&W negative and Kodachrome slide film are perhaps the least affected by age.
B&W film is basically silver crystals on an emulsion and, if developed/stored properly will last a few lifetimes. All of my B&W film from 20+ years ago still looks like the day I put them in sleeves.
Kodachrome slide film is this way because it is an additive-type film emulsion where the color dye is in the development chemicals, not on the film itself. It has been around since the 1930’s and I have read reports that 50 year old Kodachrome looked like the day it was shot.
Now onto regular color slide and negative film. My stock of a few thousand of each has definitely deteriorated over the past 20-25 years. Primarily, they have shifted in color (e.g. picked up a magenta tint) and for sure lost some saturation. Now, granted I haven’t seen most of these images in over 20 years, but I would almost be willing to bet some have even lost detail and have become more grainer.
The primary reason for this is because this type of film has the photo dye directly on multiple layers of emulsion. This dye can and will fade over time. Especially considering most of the dye used on this film, and the development process it went through, was never intended to be archival in nature. As the dye fades, so does color and detail (i.e. sharpness/grain).
With all this degradation in mind, I was able to correct for a lot of it either at the time of scanning or inside Photoshop and/or Lightroom, especially the color shifting issues. I attribute this success to the file format I used for scanning which is mentioned below.
Now, onto the scanning process. I selected and was overall very happy with a Plustek OpticFilm 7600I SE scanner for the task at hand. As of this writing, this scanner doesn’t appear to be available anymore on Amazon, but other models do seem to be in stock.
I used the scanner’s native, maximum, optical resolution of 3,600 PPI (pixels per inch) for everything and selected to save the scans in the best file format that I know the scanner supported which is TIFF. With these parameters in mind, each scan took an average of 5-7 minutes to complete for color and about 2-4 minutes for B&W. Each TIFF file was approximately 50 MB in size.
I lastly wanted to note that although the times for each scan were noted above, some images took well over an hour to complete when you consider I fixed dust and scratches that the scanner could not along with the color and sharpness corrections. And then there were some of my B&W negatives and Kodachrome slides that went straight through the entire workflow process in 10-15 minutes. So all told, it took me about six months to select, scan, and fix around 800 of a few thousand images I have on film. Doing so on nights and weekends only.