If Marxists claim that useful labor is the source of a commodities value then would a commodity that has an expiration date, strawberries for example have as much value when they turn rotten as when they are fresh?
This is more of an economic theory question, however it has interesting implications about the relevance of concepts.
In the begining of Das Kapital Marx is interested in defining what creates value. His objective is to show that the investment of workforce is what creates value, in order to make a case for the idea that it is the workers ("proletarians") who create value and not the capitalists.
The mecanisms of how value gets depleted is out of his scope for this purpose.
I think most would find that the value diminishes over time. Also, the value of say a foot treadle sewing machine became less with the advent of the electric sewing machine. Assigning value based on effort solely is not part of Marxism, however. Marx wrote that the effort should be the primary factor in figuring value, not the only one.