How long do you think this game is going to last?

TL;DR Some dude got bored of fire emblem so he thinks it's going to die.

It massively depends on what are you expecting from a game like this. This is a game, where you mainly collect heroes and complete quests while doing so. The strategic element sort of shallow, but it still makes the gameplay fun and easy to just pick up and play.

I'm not a person who would want to or have time to play mobile games more than an hour a day, so I'm perfectly happy with the content that the game offers. I also believe that the devs are trying to regularly bring some gameplay innovations to the table - new weapon skills and passive skills with each new hero, sacred seals, skill inheritance, new game modes and regularly updated story content.

So I'd say that the game is definitely not gonna die and it will keep improving with each month. You're free to stop playing if you don't like it tho.

I honestly feel the same. I do respect IS updating the game and adding all sorts of things but there's these droughts that keep hitting me and I just don't be wanting to play because I don't have much to do. This pass week I've been login in just to get the daily orbs and that's about it.

Well...

http://gadgets.ndtv.com/apps/news/fire-emblem-heroes-download-revenue-ios-android-1727363

So, nope, its very unlikely to die in a couple of years -unless- nintendo and IS mess things badly, which could very well happen, because... nintendo...
FEH's playerbase is actually somewhat small, but its very attached with the FE universe (thus happy to throw some dollars at it) The only problem they could face at some point is that they will run out of (good, beloved) heroes. Perhaps they'll try some crossovers or something at that point.
And on a extra comment- I don't want to be mean, but the top players on any game aren't the ones making youtube videos... they're actually playing the game.

I don't really have the impression that it's dying at all. In fact I feel like the pace is picking up, I mean look how many events and free orbs we've had this month compared to previous months. Also, on the contrary I'm glad that things have settled down right now. Between Tempest Trials, the Voting Gauntlet, new banners, e.t.c., we really need time to catch our breath to avoid burnout. I don't want to be forced to grind hours a day for the entire month nonstop. And it gives us time to train and build up new characters, complete other quests, do IRL stuff instead of worrying about our ranks and rewards, e.t.c.

I think previous comments said more or less everything, in fact it's more likely to find a complexe / evolving gameplay on PC or console than on mobile.

But I think they are doing an OK job at bringing new content and such.

What I would like tho, is IS to break some of those "limits" they put to themselves. Especially on the PvP aspect, it would be nice to make it more controlable (real PvP or at least some global control over the IA decision (like patterns making), and why not some map vetos/sélections?).

I think improving the Online aspect of the game would be great, cooperation missions and why not on bigger maps as well. It would push players into spreading the game, create a community (or multi-account, which sucks) but well, tryhards will tryhard anyways, so they should focus on bringing fun to 4fun-players !

It will easily last into 2018. Low ranking players might start to quit now that MKV is busy with school so now they can't watch a god do his work. Curious players always come and go. It may end up like pokemon go, where curious players jumped on and left after a short while, but it's not going to die in 5 months like you say.

I just hope that as they do add more to the game they don't add more frequent TT's. It's like 3 days out and we're already getting feelings about the game here.