Per the league referee coordinator and interpreter of league rule:

This is based on the outline from FIFA. There is a "Lightning + Thunder" component. It's called the "30-30" Rule:

You see the lightning. Start counting time. If the thunder is heard within 30 seconds, suspend the Game (Practice) for a minimum of 30 minutes. Send everyone to shelter. (NOT under the trees!) If there are subsequent occurrences of "lightning-with-thunder-within-30-seconds" then the 30-minute "timer" is reset.

So, 30-30 rule seems to make the most sense. Better safe than sorry.

USSF Rules

Minimum Delay 30 Minutes - game terminated after 45 minute delay

Example #1 - Lightning is first observed at 2:00 PM, and continues intermittently for several minutes... the last lightning strike is observed at 2:15. This game should be terminated upon seeing the lightning strike at 2:15 (because after you wait 30 minutes after 2:15, the total accumulated stoppage time will be 45 minutes).

Example #2 - Lightning is first observed at 2:00 PM. No further lightning is observed. You resume play 30 minutes later (at 2:30). If at ANY time you observe lightning again, this game should be terminated IMMEDIATELY (because after you wait another 30 minutes, the total accumulated stoppage time will exceed 45 minutes).

Referee Info/General Information

+ Lightning is one of America's biggest weather killers.
+ Lightning is dangerous...it must NOT be ignored.
+ The safety of the players is always your PRIMARY concern.
+ "Lightning" is now defined as VISIBLE LIGHTNING, including ANY kind of lightning and/or THUNDER.
When you see lightning or hear thunder:
1. Regardless of how far away it may seem, your responsibility as a referee is to STOP the game immediately.
2. Advise players and team personnel to seek shelter.
3. Tell them they may not return to the field without your approval.
4. Note the time you stopped play. If they refuse to seek shelter, or they return to the field without your approval, submit a detailed Game Report.