Perhaps decades. Perhaps thousands of years. It probably depends upon why those machines were destroyed. Look at World War II. European nations and Japan rebuilt relatively rapidly then rapidly built upon progress made during the war. On the other hand, we have the decline of the Roman Empire. While we may now acknowledge that the dark ages weren't as dark as our 19th century peers thought, the western world lost the will or the imagination to rebuild at large scales (which the semiconductor industry certainly is).