What would be the harm in declining to sign this?
The people who want you to sign it won't like you as much as they'd like you if you signed it. You should try to figure out for yourself whether there's anything they think they're holding over you. Are you planning to use them for a reference? Was the internship a compulsory part of some course, and you need them to confirm you did it? Since you left after a month I suppose probably not, and you're in the clear, but it's for you to consider whether you still need them or not.
Absolute worst case scenario, and this is highly speculative, if you refuse to sign they will maliciously pursue you over some other matter (for example they might suggest that you have confidential documents, perhaps as copies on your own computer or phone that you could have used to check email during your time there, and they want it returned or securely deleted). But this is just the simple observation that if someone really wants you to do something, and has the means to harm you, then there's some harm in not doing what they want. The same observation would apply to armed robbery.
how far into the future?
Any amount of time into the future. Whatever future business plans the company has, are covered.
Why should I know future plans if my contract is active for only a limited time?
I don't know whether you should or not, but whoever wrote this NDA doesn't know exactly what information each person signing it actually does get. If some employee, in describing a task to you, were to say to you "the reason this needs doing is to prepare for the Australia launch in 2017", then you know something about the company's plans into 2017. If it happens that you don't know anything then you can't disclose anything.
Aren't those unreasonable things to ask in an NDA?
Well, this is almost certainly their standard NDA, it hasn't been tailored for you. So what would be the alternative text for them to put in their standard NDA? To say that the company's plans for the next N months are covered, but they're perfectly happy for you to disclose whatever you may know of their 2017 plans? That doesn't seem like a good idea.
NDAs tend to be pretty comprehensive, since the purpose of them is to formally state an agreement that you won't share any confidential information you might come across. Why on earth would they want to exclude their long-term plans from this, and permit you to disclose those to their competitors?
I agree that they should have asked you to sign this before the internship, not after. The request is unreasonable in that sense. The fact that the NDA itself covers their future business plans, and covers information that didn't have "Top Secret" stamped all over it at the time you were given it, both seem pretty typical. Normally the terms of NDAs are disciplined by the fact that people who don't like the terms won't sign them and won't do business with the company, so potential signers judge them on that basis. In this case you no longer have any pressing reason to sign, certainly not to accept any terms that would be onerous, but perhaps you can generate some good will by signing. Most likely it'll make no big difference either way. I've signed a number of NDAs over the years, and I don't think I've ever actually reached a situation where I've decided what to disclose on the basis of having (or not having) signed one.