Do enzymes only live for 8 hours

Biologist here. All enzymes ARE proteins. It is folly to make generalizations about the lifespan of the activity of an enzyme, because they are all different. However, they can be grouped based on their specificity, half-life (time it takes to lose half of the specific activity), resistance to denaturation, etc. In most cases, temperature has a directly proportional effect on half-life. As I understand it (from reading on the internet, so who knows?), the enzymes used in record cleaning kits are of a very tough and stable variety, called proteinases because they can "devour" other proteins. They are active in the presence of ionic and/or nonionic detergents, which completely kill other more fragile classes of enzymes. In the case of the Walker kit, he supplies the enzyme in powdered form, which is very stable unless it's subjected to excessive heat, etc, for long periods of time. So the clock does not start until you mix it with the supplied water. As to the more pertinent question of what are we doing when we clean our records withe enzymes. I wonder about that too. Possibly there are molds and vinyl-loving bacteria that can grow in the grooves. The enzymes commonly used have some activity against bacterial and probably fungal cell walls.

What's the problem? Enzymes degrade slowly over time; they don't go away suddenly after 8 hours. Different enzymes in solution will lose activity at different rates, and temperature and buffer content are other factors that will determine the rate of loss of activity. But apparently the Walker system was deemed to lose enough activity by 8 hours that he advises discarding the solution after that much time. You have to take his word on that (or not), because none of us have done the experiment. You don't need Stephen Hawkin(g)s for that.