For a product with a set shelf life, obsolescence will occur whenever units sold during the shippable life are less than the starting inventory at the beginning of that period. Varying demand can usually be modelled as a random variable following a probability distribution such as the gamma distribution (see my prior blog: A Probability Distribution for Demand Variability).
The gamma probability density function is:
where 
is the gamma function. The mean of the gamma probability density function is ab and the standard deviation is 
.