How long is unleaded gasoline good for?

I recently had a problem with my lawn mower. It is almost brand new (just a couple months old) and the motor wouldn’t stay running for longer than two seconds. Then I realized it was probably the gasoline I had in it, which was almost a year old. So, it ran perfectly fine after I replaced it with new gas.

This got me to wondering exactly how long is gasoline good for? Is it different for other type of gasoline? Does it vary on what type of machine it is running on?