Well you can risk it if you want, I kept the same compost in a large pot on my balcony for ten years - but it was growing permanent planting, not bedding. The risk with reusing compost for replenishing bedding, etc., is a possible build up of infective agents - Pansy Blight, for instance, lives in the soil, and vine weevil loves compost in pots. If you do reuse it, you must add a balanced fertiliser to it just before planting up, it will have no nutrients left for any new plants. Otherwise, distribute it over the borders and beds in the garden and use fresh compost in the pots.

Much better and safer to use new potting compost and put the old on your garden for all the reasons Bamboo has stated. The old stuff will be full of root fibres and, potentially, weed seeds too.