TL;DR: About 99% of the time it would have taken them to do it on their own. With one possible exception... (see EDIT below)
What is "steel?" Can they find the "start" button before the power plant breaks down or, in the case of nuclear, melts down? Have you ever handed a computer mother board to a child and asked them to find the "on" button (it's there)?
By the time the primitive society figured out how to turn on a hammer-making factory the factory would be falling apart, as would all the vehicles, utilities, and other infrastructure it depends on.
Entropy is your enemy
A friend of mine devoted some of his time to teaching adult prisoners to read. It took them months, and they had a working vocabulary and a tutor that knew what all the letters looked like and what their combinations meant. A person who cannot read can't learn physics by picking up a physics book and staring at it for a really long time. By the time they started working out the basics of the language, all the paper would be moth-eaten dust, all the hard drives would be demagnetized, etc. etc.
A primitive society (let's say stone age) would benefit from the buildings (a great way to stay out of the rain), basic building materials (they don't know how to weld!) and some tools (what's a "torque wrench?"). They'd get a bit of a jump with some of our more useful tools (like spanners/crescent wrenches and channel-lock pliers), but the "advances," while seeming miraculous to them, would be an insignificant advancement.
By the time the more imaginative and creative among them started realizing the world around them was more complex than they could beforehand comprehend, it would would be a proverbial useless pile of dust. Your electricity infrastructure will be gone in mere months at best. And once gone, you can't just "find the start button." Between lack of electricity and rust almost everything would be forever useless.
The technology pyramid is enormous, and the number of people that built it even more so
Knowlege (aka, technology) is a pyramid. What we enjoy today is the tip of an overwhelmingly large body of knowledge that stretches back thousands of years, touching untold billions of people.
And what many people don't realize is that it takes more than scientists to advance technology. It takes more than engineers, and technicians, and even teachers. It literally takes nearly everybody (maybe not the bums1) to provide the food and materials, recreation and education, jobs and duties, organization and structure, to make high technology happen.
So, to conclude, it might save the primitive civilization 1% of the time, but I think I'm being generous by saying so.
Raznarok got me thinking (a good thing on this site, right?). In the beginning, the primitive society would benifit by a few, insignificant things (tools, utencils, broken glass...). At the end, it would speed things up considerably (wait, you mean that machine cuts wood? What do the rest do!).
HOWEVER, "considerably" is a relative word. The technology we enjoy today was fundamentally developed ONLY during the last 150 or so years. If you assume Clarkean Magic such that the entire power infrastructure, buildings, tools, etc., did not decay, then you might reduce that 150 year period to 25 (at a guess).
BUT, compared to the thousands of years it took to get to that 125 year savings, my estimate drops from 99% to 98%. It's still pretty insignificant.
1 If you think about it, even bums have their uses in terms of social development and the supporting economic and medical infrastructure that "helps" them (that's an argument I'm not going into.) My point is, every problem leads to a solution that contributes in some small way to the pyramid.