>>13530255From zero to the stone age: takes as much time as it takes to pick a stone up from the ground. This is about 2000 BC
From stone age to iron age (we skip bronze age): knowing the techniques, you can start extracting ore, making coal and refining iron in less than one year. This is about 400 BC. Thus no more than 2400 years to reach today's tech level.
From iron age to steam age: this requires machines, drills, lathes etc (but, importantly, not computers), so that is about 10 years. This is about AD 1700. At this point you also refine copper, make electrical equipment. Turbines require finer instruments for balances and better ball bearings, say another 10 years. This is about 1930's.
Given the knowledge already is there, we need to spend most time on development making tools, less on research, so we can halve the time to 40 years to reach our times tech level.
All in all, with all the knowledge and manpower, I'd say 50 - 60 years, or two generations.
