Generally softener resin lasts up to 15 years when it is fed with city water. Things that make the resin fail are usually iron and organic fouling (coating) of the resin.
You can be pretty sure it is time to change the resin when you notice the water start getting hard sooner and sooner.
It will have lasted 7 days when new, and then 6, 5, 4 days and so on. You can try using a resin cleaner or can go through some testing, but the cost of the resin is cheaper then the service call, so as long as the water gets soft but doesnt stay that way as long as it used to, you probably need softener resin.
How long should the water stay soft after regeneration?
Run this calculator to find out
also see :
Which water softener resin do you need?
How to change resin in a water softener