It is about cycles, not years
One cycle is the door going up and coming back down. A household that uses the garage as its front door can run eight or ten cycles a day and wear out the same spring in three or four years.
Higher-cycle springs exist. They cost more up front and are worth asking about if the garage is your main way in and out.
Signs a spring is near the end
A spring near the end of its life makes the door feel heavy and badly balanced.
- The door feels heavier than it used to when lifted by hand.
- It will not stay put when you stop it halfway. A balanced door should hold.
- The door closes faster than it opens, or lands hard.
- Visible rust, or gaps opening up between coils.
- The opener sounds like it is working harder.
What makes them last
A light coat of garage door lubricant on the coils once or twice a year cuts friction and keeps rust off. Keeping the door balanced matters just as much, because an out-of-balance door makes the springs and the opener both work harder.
If your door has two springs and one breaks, the other has done exactly the same work. That is why they are normally replaced as a pair.