People have been asking that same question for 3 1/2 years on this board. Most of those posters are long gone. I would not try to time and predict when this "bubble" will burst unless you are paper trading and not using real money.
My question is when will people stop calling it a bubble? It is not it is growth.
You WILL see AMZN trading at 325 before you ever will see it at $225.
I hope my review of past history has provided you useful and helpful insight.
It is NOT growth. It is value destruction of the highest order -- destroying profit pools to knock others out of business. When the economics become clear to the outside world, DOJ will have an EASY case against Amazon (and don't think AAPL, GOOG and MSFT won't help make sure it happens) and there will be millions of mom and pop retailers that were put out of business that will see to it Amazon pays... It is (knowingly) selling vast numbers of products at a loss to knock others out of business. That is predatory pricing. Nothing more.
Regardless, the cost structure is evolving in a manner that further exacerbates flaws in the economic model... One way or the other, Amazons' real economics and truly predatory business practices will get exposed. Like NFLX, when it tries to raise prices to cover its true costs, there will be a mutiny and its brand will get crushed rapidly...