I think they'll be done by 2017, way too late.

3DS is going to be shipping sub-10 million units by next year. And Wii U is never going to be a big seller.

Mobile/smartphones are a bigger threat than Sony/MS ... I think people are still in denial about this, but kids are jumping off Nintendo's bandwagon every day.

Really Nintendo needs a new handheld for 2015, even 2016 is fairly late. 2017/18 would incur a lot of damage to Nintendo's business and mindshare IMO. Tablets and phones are taking the 3DS to school and beating up on it, Nintendo needs to send reinforcements sooner.

The 3DS is starting to decline noticably in 2014 ... it won't make it to 2017. 

It's basically impossible to tell. The new 3DS is basically an insurance policy so Nintendo can float or dump it as they see fit. A successor could happen in 2016 or Nintendo could hold out to 2020.

I imagine that Nintendo is planning to launch the Fusion by Christmas 2017. If the Wii U has a really great 2015 and 2016 that may get postponed a year to milk the market, but that's not likely.

So between 3 and 4 years, probably closer to 3.

If they still can't attract strong sales in the US after Pokemon X/Y, Mario 3D Land, Mario Kart 7, Super Smash Bros. 3D, Zelda: LBW, Zelda: OoT 3D, Kid Icarus, NSMB2, Animal Crossing, Resident Evil Revelations, Kingdom Hearts, Luigi's Mansion 2, Nintendogs + cats, Brain Training, Mario Party, Sonic, Monster Hunter 3G, Bravely Default, Yoshi's Island, Kirby Triple Deluxe, etc. etc. etc.

And a huge price cut to the original 3DS model early in the life cycle + 3DS XL + 2DS (discounted to $99 for periods of time at many retailers) ...

I'm going to go out on a limb and say it's never going to happen. It's sold "OK" in the US, but will never be a huge winner here. Too many kids/casuals have ditched Nintendo for smart devices here. At least in Japan there are people who basically use the 3DS as their main home play console (basically, as no one is buying the Wii U or PS4). But the US doesn't have that dynamic. 

2016.

Ya I guess 2016 isn't too out of the question, it would give New 3DS 1-1.5 years in the spotlight. The only problem is if they release it spring 2016 then they would have to announce it in mid 2015 which would essentially kill any New 3DS momentum. I'm thinking they will announce it mid 2016 and release late 2016/early 2017.

Nintendo has enough lined up to support 3DS through 2015 but ur right that after that point there really isn't a whole lot to push hardware.

New 3DS is something more for Japan IMO (part and parcel why it launched there first). It's to prevent 3DS sales from completely collapsing YoY ... because imagine what 3DS sales would look like in Japan YoY if they hadn't launched that model ... they would be down 2-3 million YoY I'd say fairly easily, which would've been a disaster. 

No one in the West is really going to give a sh*t about browsing the internet faster on their 3DS or an analog nub for Monster Hunter as MH is not that popular here to begin with. That's not going to make the kid who wants a new iPad mini for Christmas go "wow".

2DS was the revision Nintendo was hoping would take off in the US/Europe (cheaper, more kid friendly model) but it really hasn't.

I know this probably won't happen but 3DS could really use a $50 price cut to all models.

2DS-$79.99
3DS-$119.99
3DS XL-$149.99

It should be announced next year. Sales of the 3DS are dropping way too quickly.

Honestly New 3DS isn't likely meant to attract a ton of new users, Nintendo is probably hoping that a large number of current 3DS owners upgrade to the superior model. Like u said 3DS isn't the sales beast that previous Nintendo handhelds were so they need to get as much money from existing owners as they can.

Just because Fusion launches, doesn't mean the 3DS will be phased out immediately either.

New 3DS models will phase out all current 3DS models and those will be around until 2018.

I can see this

Fusion (portable variant) - $219.99 MSRP launch
New 3DS/N3DS XL w/game bundled - $119.99/$149.99

Come May/June 2016.