Hi awoods,
There isn’t an “expected lifetime” for pocket coils as a whole because there are hundreds of versions of pocket coils that are all different from each other but the amount of steel in any innerspring system (along with tempering and any precompression of pocketed coils) would generally have the biggest effect on durability relative to other types of innerspring of the same type. The amount of steel would come from a combination of the height of the coil, the number of turns, the gauge of the wire, the shape and diameter of the coil, and the number of coils in the entire innerspring. Having said that … while pocket coils as a whole would generally be less durable than innersprings that use helical coils to connect the individual springs together if you were to sleep on them directly (or if there were only thinner layers of foam above them or with higher weights where the coils would be compressed more deeply and more frequently) … the support layers of a mattress (such as a pocket coil or other types of innersprings) aren’t usually the weakest link of a mattress which is usually in the quality of the comfort layers above the pocket coils.
There is more about innersprings in this article and in post #10 here and there is also more about the many variables that can affect the durability and useful life of a mattress relative to each person in post #4 here but in most cases if you are looking for a more durable mattress I would focus on the quality and durability of the materials above the coils and I would focus on the feel and performance of the deeper support layers and how they contribute to the mattress as a whole more than on their durability.
Sometimes pocket coils can have issues with glue degradation or with shifting but again this would be a minority of cases and in the case of pocket coils made by a reputable North American manufacturer (Leggett & Platt is the largest manufacturer) … for the most part the materials above the coil will have the biggest effect on durability.
Phoenix