What is the lightning overlay refresh rate?

The lightning shown on the Radar overlay is updated at the same frequency & time as the Radar image. The app checks for lightning information every 3 minutes, but the Radar images themselves are updated every 5 minutes. The app checks lightning more frequently in case you had ForeFlight in the background, so it can get the latest image right away.

If you choose the separate Lightning overlay, that is updated every 5 minutes and includes all strikes in the previous 5 minutes.