An easy-to-remember rule to follow if you see lightning - it's called the 30-30 rule.
If you can count thirty stalls after you see lightning, then the lightning is far enough away that you can continue to play your game. If the lightning is 'closer' than thirty stall counts, you should immediately discontinue play and seek appropriate cover quickly.
The second 30 refers to 30 minutes. You should wait for half an hour until after the last lightning is observed before resuming your game.
Please note that the WODS policy on Lightning and Adverse Weather can be found here.