I've been with Pokemon since it came out.
I was one of the first to line up in the store when it opened to buy the very first 2 games.
I'm almost 20 and I've stuck through 4 generations of Pokemon and am eagerly and excitedly awaiting the 5th generation.
I care not for those fans that leave because they don't like the newer generations.
I care not for those fans that bash the newer generations or for the newer fans that bash the older generations.
As cliche as it sounds, one thing rings true in my mind...
I have and always will be with Pokemon, right to the end.
Where people leave when they see a new generation, it makes me stick by it harder.
I don't understand people when they leave for ridiculous reasons such as a newer generation of Pokemon.
Different Regions, Different Myths, Different Pokemon, Different Ideas, Different Stories.
Nothing to complain about.
I hate to sound like a fanboy but I suppose with this, I am proud to sound like one...
To deserve a spot in place when Pokemon is at its best, you must be willing to endure with it even at its worst, no matter how hopless it may seem, even though it doesn't seem hopeless at all, quite the contrary, each newer generation brings awesome in my eyes.
But that's just MY opinion.
To be perfectly honest...
My answer to your question: FOREVER.