Microsoft is saying this year is XBox 360 all the time, and a big year, and don't expect any announcements.  This does NOT mean that the next Microsoft console won't come out next year, or the could do announcements for it next year also, with no talk this year because they don't want to hurt sales.

But if not this year, when will the announcement be?

I would say that Microsoft could announce at next year's CES, and then have a release at the end of the year, and make next year the year of the next Microsoft console.  However, I am not going to bet this is so and am curious to see what others have to say.

E3 2013 (June 2013)

I can see them announcing new hardware at E3 2013 to come out the first half of 2014 or even before the year (2013) it's over.

Somewhere between June 2012 and March 2013.

June 2013 at the very latest I think.

Depends on Holiday 2012 sales.

If sales aren't as much as MS are expecting post-cut, I'd say it could be shown as early as CES 2013. If sales are alright through the Holidays and early 2013, then E3 2013 would be my best guess, ready for a Holiday 2013 release.

Microsoft's last CES was this year(2012). They won't be making an appearance in 2013.

When do I expect an announcement? E3 2013.
When do I want an announcement? E3 2012.

E3 next year with a fall 2013 release i severely doubt it will be announced before gta5 releases

If sales are low, they won't announce a new console, it'll kill sales. 

There's a computer company that existed in the 1980's that made the mistake of announcing their next computer well before they were expecting to release it.  The announcement killed sales, because no one wanted the older computer when the newer one was so much better.  The company literally ran out of money during the development of the computer model everyone was holding off for, and it went out of business.

This is why companies are so cagey about what they're up to.  Sure, they don't want to tip off their competition, but they also don't want consumers to decide not to buy not rather than wait for the next generation.  Ideally, everyone would like to do a stealth release like Microsoft did with the Xbox 360 S, a total suprirse to everyone.

The Wii U was announced way too early, imho.  Granted sales were dropping, but I believe they got worse after Nintendo's announcement of the Wii U. 

E3 2013