It depends on how you use the sofa, but for most homes, washing or changing sofa covers every 3–6 months is a reasonable target. In houses with kids, pets, or people who eat on the sofa regularly, you might need to do it more often, maybe every 1–2 months for the most used couches.
Pay attention to visible signs: darkened armrests, food spots, a general dull look, or smells that don’t go away with simple airing. Removable, machine-washable covers make life much easier. If your sofa doesn’t have those, consider using throws or slipcovers in busy areas. They can be washed regularly while the main upholstery stays cleaner longer.
Also remember to vacuum under and between cushions now and then. Crumbs and dust settle there quietly and attract insects over time. If you have allergies, keeping fabric furniture cleaner can make a noticeable difference.
You don’t have to treat the sofa like a museum piece, but giving it a proper wash cycle a few times a year keeps it feeling fresh and extends its life.