Detailed Answer
Gutters rarely fail all at once. Most homeowners reach the replacement threshold through a gradual accumulation of symptoms that individually seem minor but together indicate the system is past useful life.
Sagging is the most visible sign. Gutters should maintain a consistent slight slope toward downspouts — typically 1/4 inch per 10 feet of run. Sections that visibly bow downward or pull away from the fascia have either failed hanger connections, fascia rot that no longer holds fasteners, or accumulated enough debris weight to deform the gutter profile permanently. Isolated sag can be repaired by replacing hangers, but widespread sagging across multiple sections indicates systemic failure.
Rust, holes, and visible cracks point to material failure that can't be repaired long-term with sealant. Rust-through on galvanized gutters is terminal; aluminum doesn't rust but can develop stress cracks at joints and corners after years of thermal expansion and contraction. Any point where water actively drips from the gutter face (not from the lip during heavy rain) indicates a hole or joint failure.
Fascia damage is both a symptom and an accelerant. Overflowing or leaking gutters saturate the fascia board behind them, causing wood rot that spreads into rafter tails and soffit over time. Soft, discolored, or peeling paint on fascia adjacent to gutters is a reliable indicator that the gutter has been leaking at that point for an extended period. Replacing gutters without replacing damaged fascia creates a problem — new gutters can't be properly secured to rotted wood.
Repeated downspout clogs and overflow despite regular cleaning often signal gutters that are undersized, improperly pitched, or have deformed sections that create low points. Rehanging with proper slope fixes some cases; others need replacement.