You do not re-tie a leader because it looks scruffy. You re-tie it because you can feel a groove in it, and the groove is where it will break.
What wears first on a sliding knot
Fluorocarbon wears from the outside in. The coating is the first thing to go, then you are left with the bare core, and the core is what frays and eventually parts. On a sliding knot such as the FG, the two lines grind against each other every time you cast, so the wear is concentrated exactly where the knot sits.
Three checks before you trust it
- Slide the knot along the line by hand under light tension. If it still slides freely and locks when you pull, the knot itself is fine.
- Run a fingernail down the section the knot covered. Smooth means the coating is still there. A groove or a rough flat spot means the coating is gone and that section is now your weak point.
- Look at the tag end. A trimmed end that has frayed into a fuzz is cosmetic. Trim it flush and it stops looking like damage.
Why it matters more on fluorocarbon than on braid
Fluorocarbon is nearly invisible underwater and it sinks, which is why it is the right choice for clear water and wary fish. It also has a soft outer coating that abrades. Braid is tougher and handles abuse better, but it is visible in clean water and it stretches, so a bite feels softer and a fish can throw the hook from it.
A good middle ground is the 8-strand PE braid, which keeps the low stretch of a solid braid while running thinner than a classic woven braid. From 0.6# to 8.0# covers panfish through bass without changing spools mid-trip.
How often to replace
Re-tie every trip if you are fishing structure, and check the leader in the evening rather than at dawn. A leader that has been under load all day is telling you something, and the cheapest time to hear it is before the next cast.