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By GPL, they're only obligated to release an offer that allows costumers to request the source code. They can still keep the source "closed" by default.

No they have to actually fulfill that request too. Once someone has the source they are then free to distribute it.

I don't remember if this is in the original text, but is there a time constraints on distributing the source on request?

If a user asks for the source, and the distributor says "sure" and then delivers it 12 months later, have they violated the license?

It has to be the source of the distribution the user currently has a copy of. So they can't just say "sure" and then wait until the next public release. I'm not sure about timeliness, though.

This is why most of Android outside of linux kernel is not GPL.

From other discussions, it sounds like they are shipping the copyleft source on time, only the permissive/pushover licensed stuff gets delayed source releases.