Once the roots fill the container, it's time to repot....either by root pruning or potting up to a larger size container. The soil is replaced at that time.

I have large containers, about 25 gallons. The potting mix lasted about 5 years before a decline in the blueberry bushes. At that time the soil seems to have collapsed to about 2/3 the volume, limiting the oxygen in the soil. I will be repotting this year. I am sure it depends on the climate and amount of fertilizer that is applied. Mine also went though a flood in which about 10 feet of water covered the plants 2 years ago. That didn't help the life of the mix.