What is your Mileage?
I had the same issue with my 2005 ZX3. I had a 150K miles and the gauge was still in the green but I new I had to replace it.
If yours has under a 100K and the gauge is still green I wouldn't worry about it for a while. I saw no real degradation in fuel economy.
I ended up putting a FS CAI (long tube) system on it and am very pleased.
Hope this helps?