Based on the life of the DS and 3DS's current furtunes, I'd say the 3DS's successor will come out no earlier than Holiday 2016 and no latter than Holiday 2017. Probably alongside the NX console, but things can vary. And of course, Nintendo supported their older handhelds after their successor were released. The DS was released in 2004, but the GBA got a fair number of games in 2005, like Pokemon Mystery Dungeon, Mario Party Advance, Mario Tennis: Power Tour, DK King of Swing, and Drill Dozer. Later on, after the 3DS launch, the DS received Kirby Mass Attack, Pokemon Conquest, and Pokemon B/W 2.

So my guess is about two years or a bit more of new notable 3DS games.

Not much time left. The games are thinning out, until there's none left, and we wait for the next handheld... : X

I'll give it 2 years at max.

im hoping for pokemon Z, unity support for lots of indie ports and some cross platforms games among NX and 3DS and even mobile and 3DS.

2018 would be my guess.

The 3DS is still Nintendo's far and away best selling hardware this gen, even if we remove Japan, in the US and Europe, the 3DS should sell around 18-20 million each in those markets ... which is more than the Wii U worldwide.

Nintendo consoles are the ones not pulling their weight in this equation, even if we factor in the massive challenges traditional portables face in today's market because of smart devices.

In terms of their best markets to worst, for Nintendo it's probably like this

3DS Japan > 3DS North America >= 3DS Europe >> Wii U North America > Wii U Japan > Wii U Europe

Until the next handheld is released.. so probably in about a year or 2?

Probably another year, at least. The support for it is still surprisingly good. I bet we'll even see the next mainline Pokémon for it.

2 years tops.
Next year will be it's last big year with a large number of titles coming out, some of them very highly anticipated.
Fire Emblem: Fates
Pokemon Z
Youkai Watch 3 in Japan, and depending on how good the 1st game does in the West, possibly Yo-Kai Watch 2.
Bravely Second, Mario & Luigi: Paper Jam, throw in Federation Force for extra , although that may be pushed back to 2017, I'm willing to predict.
Then 2017 will be a mostly dead year, with the only possible big titles I can see being released for it at that point being Pokemon Gen 7.
Whatever they might have had planned for the 3DS at that point, same with whatever they had planned for Wii U, I think has been moved to NX.

I think the next handheld will launch late 2016, it still has Paper Mario, Fire Emblem if, and Monster Hunter X + unannounced stuff next year

One to two years imo.