When is the usual update for macs?

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I'm looking to buy a MacBook air.
I had MacBook pro back in 2010, sold that and got a mac mini.. Now want a laptop again. The mini is great but I'm only using it as a media player( connected to main tv) not getting the use as an actual computer so that's my reason for selling.
I fancy the air for a change as I've already had the mac pro.
If say retina display laptops were around the corner then I'll wait
Any ideas on next apple event for laptops? ...do they do that? Or are they just iPhone/iPad reveals?

Cheers anyway

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I had a look at the guide before. Just with all the '2012 mac updates' I've seen on here, was thinking maybe it was the same every year, like iPhone/iPads release.