I don't particularly care how other people handle it in their own campaigns, so I don't have an answer to the "when should" part of the question.
Were I to be running a campaign in the system (I'm currently playing in one that uses the final public playtest rules), my answer to when would my players get their first +1 longswords is "never".
I've never been fond of +n items, and given that the system is apparently designed so that their inclusion isn't expected or required, I see no particular reason to include them.
That's not to say that there wouldn't be magic items, even magic longswords. It's just that none of them would be +n. All those little quirks you can randomly generate to make a +1 sword less boring? If I want to use those, I can simply use them, without mucking about with the math.
A sword made of flame, which provides illumination, heat, fire damage, and the ability to start fires is magical with or without a number attached to it. The number is superfluous.
When would the characters get their hands on magical weapons or items? It's something that's going to vary, depending on what seems interesting and fun to me and my players at the time.
I might start one campaign with each of the PCs having a magic item at level 1. Or getting them might take most of the campaign.
Or I might make it so that each magic item in the world is already known and famous, and if you want one of them you have to go seek it out.
Maybe each item needs to be earned. Or maybe they're a dime a dozen, found under every rock, in every treasure hoard, in every enemy's hand.
There's no blanket approach that would work for me all if the time.