Answer....that depends. :-)

I see that most water softener salt is, simply, sodium chloride. Some is potassium chloride. Some are other salts. We'll assume yours will indeed be sodium chloride. It's cheap and effective.

ETA: 103 mg/L easily translates to 103 PPM. Which is a fair clip, actually. /ETA

If rarely irrigated and flushed through by ample rainwater, practically nothing is the result. So if, like me, you're in the habit of irrigating once or twice a year, absolute maximum, then don't worry about it. Even sodium-sensitive plants won't notice that.

If you happen to live in a semi-arid or arid area and irrigate all the time, irrigate in smaller amounts that never wash the sodium below the root zone, and never receive ample enough rainfall to leach the sodium out of the soil, you'll eventually end up with a dead, sodic soil that won't grow much except sodium-tolerant marsh grasses. And then, not even those. But maybe you can manage a few things that tolerate areas like the Dead Sea.

From the description, "not regularly" means your impact is towards the former. I'd be less inclined to worry about it, but would make sure to apply more water less often during dry summers to avoid sodium buildup.

Pro tip: Turn off your water softener when you're irrigating the lawn and simply let the hard water hit the lawn. The lawn really doesn't care about that and would much rather have the calcium than sodium anyway. Also, it's far cheaper in terms of your water softener salt. You can certainly turn on the softener temporarily if you need to use the water yourself, but why pay to soften hundreds of gallons that hits the lawn and doesn't do any good?

That's been my solution. Or I simply let my lawn (bluegrass) go dormant for the summer--but I have a thing about throwing potable water at the lawn, which I understand that many people are fine with (and that's OK).

It's best, when watering, to do so deeply and rarely anyway. One inch at a time, once a week, will encourage the deepest roots in your lawn--if the lawn you have is inclined to grow deep roots and your climate allows you to go the full week. Watering small amounts very often simply encourages shallow rooting and, overall, far more water usage since the water never penetrates that deeply anyway.

Again, it depends--we tend to recommend a full inch of water at a time, which is a whole heck of a lot of water. But if it rained that day and you're irrigating to enhance that, you'd be watering less.

And it depends on the size of your lawn. Postage stamp lawns won't take much, of course. My lawn, at 10K square feet...quite a bit. And some people have huge lawns. Let's not discuss my gardens atop all this...

Salt at that quality runs about $0.20 a pound at decent bulk, more if you order in large quantities. At 100 PPM, it's one ten-thousandth of the water weight. One inch of rainfall is just about 0.8 gallons per square foot, if I'm doing the math off the top in my head correctly, or seven pounds almost exactly.

(OK, America, can we get with the friggin' program and go SI? Pounds, feet, this just sucks).

NEWay.

Working with a 10,000 square foot lawn, or 70,000 pounds of water per inch of water per watering, that's seven pounds of salt at 100 PPM.

This is approximately one frick-load of salt, in other words. Or $1.40 wasted per watering, and that actively damages the soil (neither sodium nor chlorine are helpful to your soil, and, in fact, both are actively damaging).

I would advise bypassing the water softener.

I would very strongly advise bypassing it. It's not that seven pounds over 10,000 square feet once will destroy your soil--it won't--but that doing it repeatedly will slowly reduce the health of your soil over time without it being flushed out by a lot of rainfall.

Sodium can be dispelled using gypsum (calcium sulfate), but...that costs money, too, and effort, and time, and soil testing, and...

I don’t know what’s involved, but we had a separate meter installed for outdoor use and that saved us a lot on our water bills.

Yes, water deeply once a week if dry...or don’t water at all. One way or the other to keep your lawn alive.

One side point, if you wash your car at home you'll appreciate having access to a spigot with softener water. Untreated water at my house leaves water spots all over the car.

Thanks for the feeback everyone. I don't think it's going to be a salt cost problem. In the last year I only had one month where I could see a spike in my water usage due to watering. I was worried about the salt impact. I've looked again at my basement and can reasonably get to two of my spigots to bypass the softener. The third one (interestingly) that I would wash my car with is hard to get to. I think I'll leave that one and do what I can to put a new one on that side of the house in the easiest location.

Pre-rebuild I'd replumbed the outside tap used for car-washing to softened, and was amazed at the difference. When designing for the rebuild, I insisted not only in softened for the garage (car-washing) taps, I also had hot run out too. That may be a bit overkill, but whatever. :-)

The other outside taps are unsoftened. But we rarely water our grass here, there's just too much lawn to make it viable.