When did the Inception start?

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 hypotheses on that, but I think that 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

What’s happening in the movie: Saito says he’ll clear Cobb’s name if he takes the job. He asks Cobb to take “a leap of faith.”

“The Entire Movie Is a Dream” Argument: The phrase “leap of faith” occurs over and over. It’s an artifact of Cobb’s subconscious.

Nolan’s Comment: “I don’t think I’m going to tell you about this.”

Source

I'd like to see the movie having - reality and dreams. There are two major dream sequences, the first one which has a 2 dream sequence where they are trying to extract information from Saito. The second one is the rest of the movie - the multi dream sequence where they are trying to incept the idea into Fischer's mind. The second dream starts after the scene in the plane. The plane is real world, the dream starts on the streets.

Here's an illustrated explanation of each of the dream levels and movement between them, kicks etc: Inception Explained With Illustration

The entire film was not a dream. The movie started when Cobb and Saito are in that dream where he proposes that he can extract information and protect his subconsciousness and tells him about the safe where Saito looks at the wall. This is where the film started and when Cobb reaches America to his kids and spins the totem, that is where it ended. The totem he spins belonged to his wife. He did not spin his own totem.

As explained the concept of infinite steps, the movie is also infinite to define a start and an end. All that we can define is an absolute present, which is supposed to be the time where Cobb accepts the offer to implement inception i.e., the point where we can move to and fro to understand the flow of the movie presentation. From that point, if you move backwards, you reach the end of the movie through the starting of the movie, where Ciato is again shown as an old man, as if he was shown as old man in the start of the movie. There are 4 perspections in the "story-telling" process of inception. 1. A director's perspective 2. Flash back of Cobb 3. Levels of inception 4. Imagination or feel of characters (Mainly of Cobb) The scene where the totem keeps on rotating is just a mode of screen presentation. If you consider that all the movie is a dream, then the totem stops rotating when Cobb meets the drug dealer to be a part of inception. Totem conception to decide that the entire movie as a dream is contrary in these two scenes.