After a few months the nutrients from any compost will be gone so when planting up afresh you'll always need new stuff.
The old/used/spent compost can be dug into your ground and soil but it won't have any nutrient value left, it'll add a bit of bulk though.

I agree and I put my old spent compost on the garden.

There is a way you can reuse it - you stir into it granular feed, such as Growmore or Vitax Q4, adjusting the quantity depending on the size of container. BUT, and its a big but, you run the risk of infection in any plants you use - compost in containers may harbour vine weevil, root aphids, fungal pathogens and all kinds of nasties, so it is a risk every time you do it. Pansies, for instance, are more likely to suffer pansy wilt if planted in old compost, but sometimes, you get away with it, so its up to you whether you want to risk it or not. If you have a garden, then distributing the spent compost over the soil will be largely beneficial.

Thank you Louise, 1 and Drc726, for your advice, Unfortunately I don't have enough planting ground 95% of my garden is in grass and if I spread it over it I would have a problem with moss. So what you are saying there is know way I can regenerate it.

We must have posted at the same time, Johnjoe...

Thank you Bamboo I did'nt see your comment before I put mine up.

We always change the compost when changing the plants to prevent the buildup of pests and diseases in the soil. In the open ground, such problems are normally reduced by natural enemies, which may not decide to move to the potting compost, or may not work efficiently there. So used potting compost can serve again as a soil conditioner or mulch in the open garden, but is a risk to reuse in pots, and the risk increases with every re-use, no matter how well you replace lost nutrients.

Thank you Tugbrethil, what you are saying it is not worth taking the change reusing the compost in my conditioners. Just for a matter of interest what nutrients do they use in the compost. John....

Well, the nutrients lost depends on the type of compost to begin with, the kind of water you use, the weather, and the kind of plants you were growing. With hard tap water, the nutrients that run out, starting with the fastest, are nitrogen, potash, magnesium, & sulfur. With soft tap water or rain water (from a butt or from heaven) add phosphate, calcium, & iron to the list, right after nitrogen. Since knowing what is really needed would require extensive (and expensive!) testing, I would just use a shotgun approach: adding a timed release fertilizer with as many nutrients as possible in it. At least, that is what I do when I have a pot that has a permanent shrub in it, as well as annual flowers. I do do a "crop rotation" on that, though!

Thank you Tugbrethil,I think I will leave it to the experts like you, many many thanks again.