The answer to 'when to replace shut-off valve' is that you should replace a shut-off valve when it becomes difficult to turn or starts leaking. Shut-off valves are critical components that control the flow of water to fixtures and appliances, and they can wear out over time due to frequent use, mineral buildup, or corrosion.
As a general guideline, most shut-off valves should be replaced every 5-10 years, especially if they are older brass or galvanized models. Signs that a valve needs replacement include difficulty turning the handle, dripping water even when the valve is closed, or the valve not fully opening or closing. Replacing a shut-off valve is a relatively straightforward plumbing task, but it's best to call a professional plumber if you're not comfortable doing it yourself.