Due to this thread, which I found useful and interesting, I called the Culligan office where I bought mine ten years ago.  Lady there said resin in the tank should last 15 years.  I explained we were still using the same amount of salt and could hear the unit recycle from time to time, but wife and I didn't think soap worked as well as it used to.

She said our salt use was about right for where our unit was originally set, and told me how to reset it to cycle more often if we chose to do so.  I was impressed there was no pressure to sell me a new unit, and she did offer to test our water if we wanted to do that.  Since our office is 30 miles away, I politely thanked her for the offer.

We used to buy our salt from them, they hauled it to the basment and kept the tank full.  They got some employees that appeared to be on meth from the looks of their teeth, sloppy and poorly groomed.  They would leave the salt outside even if we were home and drive off quickly.  We figured we might as well buy our own salt, which we purchase by the pallet, and save some money.  It actually would have been worth it for them to put the salt in even with an increased price, but they could not service us correctly with employees we actually didn't want in our house anyway.

Sorry to get carried away with this post, main point was the resin should last 15 years.