How long do softener resin tanks last? I know that it will vary based on incoming H2O, but approximately, adn how will I know?
Trying to plan wisely.

That's a tough one, I've seen them go as long as 15 years and as little as 6 months. You pretty much hit the nail on the head about the incoming water, that as well as capacity seem to have some effect. Better Water and US Filter put their softeners post carbon on the theory that chlorine is the big factor. I have yet to see a difference. keep an eye on the head, I've had a lot more issues with them than the actual resin.

what size tank do you have?

Are you asking about the softener tank or the softener resin? A softener tank should last 15+ years as long as you don't over pressurize it. Some tanks may be susceptible to sunlight breaking down the plastic.

If you are talking about the resin, yes, it depends on the incoming water. A softener will remove most cations (Ca, Mg, Fe, Mn, etc) but mainly Calcium and Magnesium are removed during regeneration. If you have high concentrations of other cations, over time, they can foul the resin. You can periodically run a resin cleaner through the softener to remove many of these other cations. Also, if the softener is in front of the carbon, chlorine can break down the resin over time. Someone once told me the resin should last for 10 years and to subtract a year for every ppm of chlorine in the water.

Good info, thanks. Is the hardness test strip basicly what will tell me when it is going bad?

Mine are before the carbon tanks for portable Ros. They are the .25cf tankjs.

There are a couple of indicators that will tell you when the resin will probably need replaced. The two most common are that it just isn't removing hardness from your water and the pressure drop across the tank is to great.

The one I'm most familiar with is a pressure drop. Years back, Gambro used a certain type of softener resin in their new installs that wasn't up to the task. After about 6 months to a year, the resin basically turned into a gooey mush that was killing the flow rate through the tank and causing a >30 psi pressure drop as well as not removing hardness. We rebedded the tank and it's been running great ever since (about 4-5 years ago).

However, in many cases, you might find that there is a problem with the regeneration cycle so I'd check that before I start tearing into the tank.

I'm rambling... I'll shut up now.