So I don’t know if the question was posed by an engineer or by a non-engineer, so I’ll answer it for both and tailor my answer to each’s general knowledge base.
If you are an engineer, your job specifications should provide you with the answer. Whether your project allows you to put a load/traffic on your slab early and at what concrete strength that can be done should be specified. Check to see if there are early f’c allowances. I work in Illinois: Our state’s specifications allow high early strength pavement patches to be opened at 3200 psi.
If you are a non-engineer (and that’s my guess…), you likely have a sidewalk, or a driveway that you are going to install and you want to know when you can drive on it, so the technical gyrations I noted in the paragraph above won’t mean much to you. With normal concrete, it will take about 5 days to get to a strength where you can drive a car on it, but that assumes a couple of key points: Temperature is a key factor as concrete – The warmer it is, the quicker the concrete strengthens. But if it’s cold, you need to add extra days to get strength. Also, concrete needs to cure: In layman’s terms, that means “cook.” The contractor should have either covered your concrete with plastic or sprayed curing compound on it. This seals in the moisture and allows it to continue to, what we call, hydrate. Once the concrete dries out, it stops hydrating whereby slows/stops the strength gaining process.
Hope this helps. Cheers!!