If there are no other factors involved, then mechanical seal life is limited by how long it takes for the carbon face to fully wear. However, mechanical seals rarely wear out because of old age and usually fail due to some other reason well before the carbon wears excessively.
There are many unknown factors that influence the actual service life of a mechanical seal, making it almost impossible to predict. Typically, it is only possible to compare empirical values from similar applications. For example, a seal operated correctly on a clean, stable media such as a light hydrocarbon may have a service life exceeding five years, whereas a seal operating on a highly viscous or abrasive product may last for just a few months.