TL;DR 2000 years
The only historical example that we have shows that going from late iron-age technology (the rest describes why iron age) to smartphones has taken around 2000 years.
Progress isn't a priority
The immediate result of such an event is that most of humans will die. Ignoring the other possible effects (housing - in many climates lack of proper housing means death during the first year), medicine, conflicts) the agricultural efficiency would drastically fall. If we're unable to get up a running industry to make a very, very large amount of tools we wouldn't even be able to harvest the existing ripe crops. We would be unable to quickly match medieval technology due to lack of large numbers of load-pulling animals - draft horses and oxen. Even if we'd immediately get up our technology level up to year 1800, that would allow us to feed a billion people, leaving the other 6 billion to starve.
That magnitude of death will not happen easily or peacefully - it means that immediate survival will be a complete priority and preservation of long-term knowledge (as well as anything long-term) will be an unaffordable luxury.
Most of the skills are immediately useless
Contractors know how to build a house using tools, concrete, screws, drywall and straight and even-sized planks - none of which exist nor will exist soon. People who know how to make screws and tools know how to make them using machines and metal that doesn't exist and will not exist soon. People who know how to make metal and alloys know how to make them using machines that don't exist and a large quantity of ore - that needs to be mined and transported using nonexistent infrastructure. The currently available sources for mining ore generally are miles deep and are unusable without modern machinery.
This goes on for pretty much everything - in an USA-like society, almost the only useful skills will be ones that are used as part of unconventional hobbies - recreating stone-age tools, open-air fireplace blacksmithing, pottery and basket weaving.
Progress comes at stages
You can't start modern society before having a lot of functioning industrial-age tools and processes, no matter how good knowledge you have.
You can't start industrial age manufacturing before having a lot of functioning iron age tools and processes, no matter how good knowledge you have.
You can't even start iron age manufacturing before having a lot of functioning stone age tools and processes, no matter how good knowledge you have - you'd need charcoal which requires felling of decent-sized trees, you'd need to obtain ore which will also require tools, and you'd need to survive which would require housing, farm tools and weapons.
If you're unable to make the first tools at scale - millions and millions of them - then your society won't be able to progress beyond that and your knowledge of industrial processes will be irrelevant.
Underdeveloped societies will be leading the pack
I would expect that societies which are currently underdeveloped would fare the best in this scenario. They are less dependent on infrastructure, and the skills they have are more useful in both immediate survival and in recreating low-tech infrastucture. Also, they often are located in places more suitable for survival. Places like Canada or northern Europe simply aren't habitable without any technology, you'd need at least a supply of good stone axes for firewood, spears and a lot of huntable wildlife to enable survival of small bands of hunter-gatherers there - totalling thousands, not millions. After such an event, sub-Saharan Africa would clearly be in a better position. The immediately sustainable level of technology will be the one that currently some nomadic herder communities have, since they will be pretty much the only communities that will be able to keep their food security, lifestyle and avoid absolute chaos.