As Grant Ed already stated it's not a gradual change. Engines usually run the fastest right before they essentially, "let go." Up until that point they will be very consistent and the only tuning that will be needed is to compensate for changes in weather (temp, humidity, pressure...). When the critical turning point happens your engine will become very erratic to tune, throttle response diminishes, the engine may seem like it's running rich and actually be very hot... and the list goes on. I have seen in my engines in as little as a couple tanks an engine go from running perfectly, to one that won't run for more than 2 minutes without flaming out and generally running horribly.