(Edit: I misunderstood this next part. It's potencies here are averaged over that period of time. Most of the impact comes at the begining.)
After 20 years it is 86x more potent than co2 and after 100 years it is 34x more potent. It never becomes less potent than co2 because it eventually becomes co2.
The Methane being leaked is 86x more potent than co2 over 20 years and 34x more potent over 100 years.
At 1% leakage todays natural gas emissions don't gain a climate advantage over coal until around 2100. At 2% leakage todays natural gas emissions are still significantly worse than coal in 2100.
https://www.technologynetworks.com/applied-sciences/news/glo...
http://theconversation.com/the-us-natural-gas-industry-is-le...