PLEASE do not take offence to this but you will NOT get that magic number of years that cushion last because it does not exist. We can certainly guess at it from experience but cushion has been evolving since our game first started and continues today.
everyone of the guys on here can attest to the fact they have seen cushion 40 years old and still played fine and on the other hand have seen cushion a year old and go hard as a rock. Icon will fall in the middle.
I believe other then just cheap materials there are other variables that cause cushion to go bad such as heat. we recently had a customer that had one rail go bad. Directly over that one rail was a heat register blowing directly on that one rail. I have seen tables go in uncontrolled climate control with good rubber only to come out with bad. When purchasing a used table you really have no idea if any of these conditions existed.Right?
The bottom line is you definitely have to factor the cushion in to the overall price of the table but cushion is a wear item. I would not skip buying a 66 vette because the tires were rotten!