This article on Mount Elbert showing ominous clouds coming has this:

There’s a 30-30 rule for lightning. Every five seconds between seeing a bolt and hearing thunder indicates about a mile between you and the lightning, Gratz said. So the first 30 in the rule indicates the minimum safe distance between you and the lightning — 30 seconds, or 6 miles…The second 30 in the rule is to wait 30 minutes after hearing the last rumble before venturing out again.