How long does it take a fossil to form?

I have a Koi pond that I built last summer, and when I went to purchase the rocks I purchased one of the flat rocks and it came with some sort of fossil on it. It looks to be like some sort of flying insect. How long does it take a fossil to form? Is it worth taking it in somewhere to get it looked at?

Here is a video I uploaded of my pond, at about 22 seconds I took a shot of the fossil rock as well as at the end of the video