If you look after them, almost never! I was getting a year out of a round wick that saw daily use.
You trim them when the ends char and you get a smokey flame.
When you put out the lamp, you should always wind the wick back down until it's totally in the slot and there isn't any glowing ember on the end. Wicks are ruined by people blowing out the flame and leaving the wick up. Ideally you put them out by winding down until they in the slot and go out, or the faintest blue flame is left.
You trim them when the ends char and you get a smokey flame.
When you put out the lamp, you should always wind the wick back down until it's totally in the slot and there isn't any glowing ember on the end. Wicks are ruined by people blowing out the flame and leaving the wick up. Ideally you put them out by winding down until they in the slot and go out, or the faintest blue flame is left.