The hype will last a few weeks at most until the end of the summer, however it will not somply die out. The reason being you can do it passively. Stuck at the mall with wife or girlfriend? Catch some pokemon. At boring family reunion? Eating lunch at place.that happens to be a.Pokestop? What will die out is the hundreds of people going on all day expeditions.

Way too soon to tell.  It will be down to how Nintendo, Pokemon, and Niantic handle the updates and transactions.  They need to add more direct player to player interaction.  Trading is a good start, battling is the next step.  Enhancing Gym and Stop functions are good.  And of course, the big one is more pokemon.  Adding future gens could really make for great surges in activity.  But it's all about how fast and consistently they act. 

depends on further content

3 point 5 years, 9 days, 17 hours and 50 seconds

Depends if they add more generations, but 6 months is probably my guess

Way off.

I'm thinking 3 point 5 years, 9 days, 16 hours and 50 seconds

You already answered yourself: as long as Teh Analyst God allows it to last.   

Or maybe until all its players are killed by thugs, or, in USA, by the police too.   

It depends on a few things really. How often will new pokemon be added? When will we get the patch for trading and battling other players outside of the gym? Will there be group battles? What events will be held in game and how often?

If all of these arive in a timely manner, GO will last too long for an acurate prediction being F2P. However if they fail to add features and hold events often enough, it will fade sooner rather than later. Only time can tell for now.

Evergreen.

We all live in a Pokemon world.

As it gets colder outside, it will begin to slow greatly. Can't quite hunt for pokemon when there's 3 feet of snow on the ground.