In general, a garage door torsion spring will last between 7-9 years. This average is based off of how many cycles the door uses in a day. A cycle is when the door opens and then closes. A garage door that has 4 cycles a day will generally last that long.
If you have a higher use door, it will decrease the time the springs are able to last. A door that goes through 20,000 cycles a year may last between 4-6 years.
It is not usual for a torsion spring to break in less than a year. It can happen, but is not the norm.