http://buyersguide.macrumors.com/used to be quite good, doesn't seem to be working at the moment. It shows the time between each revision, and when it was working clearly showed that sometimes there were only a couple of months between revisions, and sometimes there was a couple of years between revisions (the 30 inch monitor springs to mind).
Most Apple products were completely unpredictable in this regard. You could perhaps argue that the release of new computers or laptops was tied to intel's cpu release cycle, which while still appearing pseudo-random in terms of when Apple does their refresh, at least gives it some sane causal tie to the real world.
Oh, it seems to have loaded. So okay, right off the top we can see the iPod classic has in the past had refreshes as little as four and five months apart, and then 4 years (2006-09) where the new models came out in September each year.
Note that with Apple sometimes what counts as a 'new model' is really minor, even just something as small as a price drop.
iPod Touch, same thing, releases in September. Nano, September.
Shuffle, updated twice a year during 2006-09 period, only updated once last year.
Mac mini, updated twice in 2009, and not at all in 2008.
Since 2003 there have only been 4 years with a single iMac update, all the rest had two or more (2003 had 3!). So 13 updates in 8 years.
Mac Pro, only updated once a year since 2003 (2003 seem like a year where Apple pushed out a lot of refreshes).
MacBook, twice a year every year except 2005 and 2010, so 9 updates every 5 years.
MacBook Pro, multiple updates in a year not uncommon, 3 updates in 2006.
MacBook Air, once a year.
The first three iPhone updates were all 5 months apart (!!!)
The slowest updating ones of that lot mini, air and Mac Pro all feel (to me) like they update really slowly. Maybe the Pro will pick up with the switch to consumer chips instead of the server Xeons. In an industry with Moore's law (price per transistor halving every 18 months) running rampant back in the 2000s yearly or year and a half updates feel very slow.
The average Apple (hardware) product (including taking all the slowpokes into account) updates every 0.700205338809035 of a year. Call it eight and a half months.
So if they release a new iPad in March, seeing another one in November (just in time for Xmas! Funny that) would not be at all unreasonable.