Hi Ed.
I'm unsure as to where you've got the idea that coffins can't be exhumed within a year of death (or should that be 'burial'?). There have been some criminal cases where the police have exhumed corpses within days of burial.
The time taken for a wooden coffin to decompose will depend upon its local environment. A wooden coffin buried in peat might never truly decompose; instead it could simply become 'petrified'. (Archaeologists have recovered some of the earliest wooden coffins, from peaty soil, largely intact).
Coffins buried in more normal soil conditions will decompose at similar rates to, say, wooden fence posts. (Fence posts are treated to slow the rate of decomposition. Coffins don't undergo exactly similar treatments but the varnish used will probably have a somewhat similar effect).
Anyway, under most circumstances, surely it's not the decomposition of the coffin that's relevant, but the decomposition of the corpse within it. If you're not squeamish, the Australian Museum (based in New South Wales) has some relevant (and detailed) information on its wonderfully-named website:
http://www.deathonline.net/decomposition/index .htm
Chris