in the event of a draw there should be an infinite amount of extra time, however every minute a player from both sides gets taken off the pitch in random order so by the time 10 mins are played you are down to 1 man each, and the rule should be that you can't play rush goalies so you can't just stick an outfield player in goal when the keeper comes off.
Taking players off is super exciting in grass root football tournaments. Some of the most exciting tournaments I have seen and in some cases been lucky to coach in, have been where normal 5/6 a side match time of 10/15 minutes has happened and the score still level so they move to a golden goal situation instead of penalties and they play like this:
1: Take a player off leaving only 3 players and goal keeper on each team then play 3 minutes more.
2: Take another player off leaving 2 + goalkeeper and play 2 more minutes.
3: Take another player off leaving 1 + goal keeper and play until a goal is scored.
Situation 3 has happened a few times at our home tournament when I used to coach. One year, on a very hot summers day in a semi final with a big crowd (since most teams were out and hence all watching round the pitch) my son was left with the keeper. He could barely walk as was exhausted and had almost given up. Our keeper got the ball to him after saving the attack from the 2 on the other team. He turned round and just smashed it from inside his own half and it went in. Got the biggest bundle of his life from all our home teams reacting to the win.