scanning - Do developed negatives lose quality over time?

I am considering scanning my B&W and color negative films. I will most likely rent a scanner for this. Many of the negatives may be some 30-40 years old.

Two questions:

1. 
   

   I wonder whether the negatives are subject to a loss of quality over years. If so, in what way? (Darker? Loss of sharpness?)

   
   


2. 
   

   Scanning of a color negative to e.g. 2-3MB JPGs — roughly how much time will this take?