You will notice the difference the first time you wash a glass and there are no spots, or step into the shower and actually feel your soap lather. Getting a whole-house filtration and softening setup usually takes a plumber about a day to install once the lines are mapped out. The best time to book the work is during plumbing supply warehouse promotional periods, which usually pop up in late winter or early spring when contractors are pushing new inventory.
Don't just call the first plumber who shows up on search results. Get at least three written quotes from local independent shops so you know what the hardware actually costs versus the labor, and make sure they include the bypass valve and drain line in the bid.
That 0% window is a hard deadline, not free money, so treat it like a monthly bill. If you still have a balance left when the intro period ends, the standard interest rate kicks in on whatever is left over.