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?
The fact that value can deteriorate doesn't change the fact that — for Marx — labor is still the source of value. Unless someone puts labor into planting, growing, and harvesting strawberries, there will be no strawberries, and thus no value that can be derived from strawberries. By contrast, if strawberries sit on the shelf too long and become inedible, that is merely labor value that is wasted: labor that is expended without producing value.
In most production contexts, wasted labor is wrapped into the final value of the object. In other words, if a farmer knows that 10% of his crop will be unsaleable for one reason or another, he will price the crop in such a way to offset that wasted labor; or if he cannot, he will produce a different crop. Most production contexts do their best to minimize wasted labor — there are significant academic fields of study dealing with workplace efficiency and human resource management — but that issue wasn't of primary concern for Marx. Marx was more troubled by the way in which companies extract excess labor value: constructing wage systems which ensure those who do the labor of production get only a fraction of the value their labor produces.
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.
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.