They don't usually. Doesn't mean they aren't this time, of course.
Last summer we got iMacs and MBPs in May, then Airs and Minis in July. Then a silent MBP spec bump in November.
Back in 2010 we got the MBP early in April, then the Mini in June, then iMac in July, the uh, "current gen" Mac Pro in August, and we waited until October for the new Air.
In 2009 it was a mess, with iMacs refreshing in both March and October, and other models were all over the map.
The only thing we can really say we know is that every day that goes by makes it more likely that a refresh will happen... but there's no way to say how soon that might be, other than educated guessing and speculation based on tangential factors, since Apple ain't talking.