If we don't, a thousand.
If there is magic, until the magician decides.
If there are dragons, never.
If you don't know what I'm talking about, then I suggest you pick up a copy of Feist's "Magician."
Actually - interesting that Amazon don't have it available for the next week - I saw it on the shelf in Australia.
Feist's newer work is still enjoyable - but it's no-where near the level of his earlier stuff.
Even with everyone alive today, the world's premier superpower (the USA) has recently let on that it can no longer make components of its current nuclear missile fleet.
http://www.domain-b.com/aero/mil_avi/miss_muni/20090310_trid...
All the replies saying "we'd know how to X" are ... optimistic.
It's a near certainty that the current ways of manufacturing steel are not what the ancients used, and even if you studied the best available guide to the ways the ancients used there would be implied and secret knowledge you wouldn't have, terms and names you wouldn't understand (or worse, seemed OK but were actually wrong). So much of what we "know" simply isn't there.
Medicine? What do I know of medicine outside drug brand names? I "know" activated charcoal can help people who have ingested poisonous substances. But I don't have any understanding of how activated charcoal differs from any other kind, what it looks like, how to make it, how to administer it, or how to make or find normal charcoal or how to store it or if there are alternative poisonous kinds.
Iron? Know what Iron Ore looks like or where it's likely to be found or how to refine it without using things you can buy at a shop? Not me.
But there's a deeper problem than that lurking. Ok, we carved stone, put likely iron ore into our clay furnace, pumped by sewn leather bellows and got a rod of metal. Still, we have shelter, and we don't need girders, I-beams, railways, canons, ironclad ships... what are we going to do with iron exactly, anyway? What did people use iron for?
Not only am I lacking knowledge of how to make things and what they were used for, but we're lacking an entire civilization within which to make use of things.
When the OP asks what it would take to recreate where we are now, what does that mean? Fabric dye wouldn't be high on my list of things to work on, nor would post-it-note glue or felt, leatherette sofas or shampoo which smells of flowers and has sea cucumber in it, asbestos, pewter, even cardboard is only useful because people post things to me in it - I don't do anything with it. With no postal system and not many things, what would we want with cardboard?
In fact, the core of what I probably want to recreate is some way for other people to do things I don't want to do, in exchange for me doing things I do want to do that other people will pay me for. And by the time I've tought myself to become a blacksmith or soap maker or wheelright or glassblower or died trying, then recreating where we are now no longer matters quite so much.
i don't need anything fancy to make use of this knowledge. i could build a house, farm some crops, and stay kind of healthy with this knowledge. could make this happen in a very short period of time if the knowledge is retained. its just a matter of reogranizing to build back up the structures of society if we still had our knowledge.
it would take much longer to relearn, because you would have to relearn the knowledge and then rediscover how to structure society. if we had residual buildings and computers, they'd be useless as they went into disrepair due to our lack of knowledge.
do we still have the knowledge but have no material technological items? do we still have the stuff but no knowledge? are we being impeded by some sort of magical force? are there dragons?