Peat based potting mix eventually breaks down and loses its structure. It ceases to be light and fluffy and becomes, well, yucky. Any nutrients incorporated into the mix are used up. Plants benefit from fresh soil, even if the container size is adequate.

Fred puts it well, nutrients in the mix don't last indefinately, after a while they're used up & need to be replaced.

Not only that, but the soil can actually become sour or rancid, so to speak. Have you ever had a plant that has grown like gangbusters for 6 months or longer, then suddenly just stops growing? That can be sour soil. Of course, if the timing is right, I'll assume it's dormancy, but if it isn't coming out of dormancy on the early end of when it's supposed to, I'll change the soil.

Denise in Omaha

I usually never repot if the plant doesn't require a larger container. I usually repot in spring or summer but it happens during autumn too. I look at what the plant needs, not what season it is. I've had Hoyas growing in the same soil and pot for years and years and they do just fine. Fertilizing regularly is better than repotting too often. If I'm ever unsure to whether a plant needs repotting I leave it be.