Weird thing about Nolan. This is his second movie in which he saves the best action scene for the middle. 

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Added on July 22, 2010, 2:08 am
I disagree with him on that part. I think how dream-sharing works is that there is a dreamer, but there is also a subject. The dreamer is in control of the dream, but it's the subject's subconscious that is being explored - remember in the Paris dream, the environment was Ariadne's but the people were Cobb's. In a mind-heist, the mark is always the subject, never the dreamer. So yes, they were going into Fischer's subconscious, but via Eames' dream.

I think... 

From the looks of it, not very long. A year or two at the most. The kids haven't grown up a lot.

HAHAHAH good spot man. precisely. saito died in level 3. after he threw the grenade remember. so he went to limbo. he was never connected to the machine ariadne and cobb used to get to level 4

(the world cobb and mal built. contrary to what alot of others think that is not limbo. it is just a recreation of the limbo world they built based on cobb's memory. why? cos they used the machine in level 3 to get to the next level. therefore they were dreaming, they didnt die)

therefore the end could never be reality

so there are 2 theories i came up with:
1. cobb went into limbo when he woke up in the plane (shot by old saito, which was a projection)
2. cobb went into limbo when he woke up at the beach (in the end saito (which was also a projection) shot himself and cobb wakes up in a plane, but that is still limbo. cos according to what they said, limbo is a stage of unrestricted dreams where anything can happen)
and yes, the time fold concept, if u go a level deeper, time is longer. exponentially longer. and injuries get better too. they're still there, but they get better. and the kicks are dulled. for eg. a kick in level 1 wont reach a person in level 2. thats y they timed simultaneous kicks which ariadne used perfectly to escape from all the dreams as she fell.

and mal. mal was a projection of cobb. same as the train, same as the kids, same as the broken glass. they're all his subconscious threatening to burst out cos he already lost his sense of whats reality and whats a dream

we didn't take limbo seriously because ALL of those who fell into it escaped it unscathed, and no one were seriously "hurt" by it

i think if the movie showed the effect of not able to leave limbo in time, then we will have the sense of urgency when Cobb mentioned to Eames that they will drop into limbo in the taxi scene