A typical LED's operating time is in the order of thousands of hours; they rarely die suddenly (unless the conditions are very severe), but they will lose brightness over time.
I would expect something similar for optocouplers. Thousands of hours may sound like a lot, but if your device is operating 24/7, it's really not. 10000 hours, for instance, is just a bit more than 1 year of continuous operation. Of course, in practice, even under 24/7 operation, the opto's LED won't be ON at all times, thus the operating life will be a multiple of this.