How long does petrol stay flammable?

[Highlander]

Well I certainly know that fuel does loose it flammable qualities,.. I was only watching a program on the telly about a month ago, where they couldnt get an old engine going, no matter what they tried or what parts they replaced,... the last resort was to remove some of the fuel and put a match to it,.... it went out as if the match had been thrown into water

but they didnt say how old the fuel was