Blessings: Lightning

Start saying the blessing oseh ma'aseh v'reishit within 2 ½ seconds of seeing a lightning flash; otherwise, do not say the blessing at all.
Note You do not need to see the actual lightning bolt--you must just see the light from lightning, even if reflected from something. 
Note The lightning must be from a rainstorm, but you may say the blessing on lightning even if it is not raining where you are, as long as you can hear thunder first. (See Blessings: Thunder/Lightning: Which First and Blessings: Thunder/Lightning: How Often)