I reuse my soil when I can. The plant, depending on how long it has been in the soil, does use up some or most of the nutrient value, but most people feed their plants too much as it is, so it shouldn't be a problem.
If you want to know the principle behind it all, look up "crop rotation". Farmers rotate crops because the plants WILL eventually use up nutrients while others will "fertilize" the soil. This is harder to do in pots, but I'm sure you don't want to grow cow peas in the pots instead, as we all would, you want to get on with the next rotation.
If you are going to start from seed in soil, I'd leave it like it is until the seedling is 2-3 weeks old before you start feeding. There will still be enough good stuff in there to support life when the seed uses up it's stored "energy". People use soilless mixes all the time, so I don't see why you couldn't use soil indefinitely.
Are you doing organic? If so, I'd recommend adding some Great White or making a compost tea.