There's a book called the knowledge. I don't think it gives a specific answer, as it depends how soft the crash and what we have available to recover with. Not

There's a book called the knowledge. I don't think it gives a specific answer, as it depends how soft the crash and what we have available to recover with. Not only does it take an enormous amount of knowledge to make all the machines that make the gadgets that make the chips, but to even have a society that can make and maintain those machines and the supply chain behind them may well require a large population and to feed that population you might need some petty modern technology. My guess would be in the hundreds of years from the stone age, assuming we remember those technologies were possible, and we bootstrapped as fast as possible. In practise, you can imagine calamity would breed religious anchors that could weigh us down another thousand years.