If your headlight bulbs were installed at the same time, the life expectancy will be fairly similar. If one goes out, the other probably isn’t far behind. That’s reason enough to replace both. Deal with it once instead of twice!
The light from headlight bulbs tends to diminish as they age. Small particles from tungsten filaments flake off and can darken the inside of the glass. The flakes can burn and create a gas that reduces the bulb’s output. A bulb can lose as much as 35% of its brightness over time. Replacing only one bulb will leave you with unbalanced lighting.