Natural gas is formed from decomposed things such as animals, plants and even peoples from millions of years ago.
There's no way to know for sure how long it took for that natural gas to form from those decomposed animals, plants and humans but it's likely it took 1 million years or longer to form the natural gas into what it is today.
Natural gas is a precious resource and takes a long time to reproduce and it is a renewable resource but it takes many lifetimes for it to be reproduced.
The natural gas we used today was likely formed over 550 million years ago.