Many have wondered about this and this may be the silliest question asked regarding the movie INCEPTION but here it is....
When did the Inception start?
Could it be that the whole movie was a dream from start to finish?
As for the whole movie is a dream thing. I found this interview of Nolan where he refuses to divulge any information pertaining to that specific question. We can make multiple hypothesis on that, but i think what Nolan was aiming it to create the ambiguity and I also think that's what made inception tick more than many movies which had relatively similar concepts
The quote from the interview goes like this