EPA’s secondary table is the language we will use: above 0.3 mg/L, iron can color the water, leave sediment, taste metallic, and stain reddish or orange. That is a nuisance standard.
Form matters. Ferrous iron can sometimes be reduced on softener resin within limits described by university extension (UNL and NDSU discuss single-digit to about 10 ppm as the zone where a softener might still be asked to help — and where a dedicated iron filter becomes the honest tool). Ferric iron is particles. Hydrogen sulfide and iron bacteria are separate problems.
Match: air-injection / catalytic iron media first on a well that stains or smells. A softener is for leftover hardness, not for “the rust and the egg smell.”