I once let my Delvac-1 stay in the crankcase for TWO years and 10kmi. The analysis came back excellent, as usual, no problems. I think 2 years would be about the limit I'd be comfortable with though. As mentioned previously, try to take the car on a longer trip (hour or two and freeway speeds) at least a couple times per year to let the condensation, etc boil out. Lots of short trips are bad for the oil, but that's one the reasons to use synthetic - it deals with it better than dino. My car gets to operating temps every time I pull it out of the garage, I never do short trips, so for me it was no problem.
Rivermaster, if you drive 45kmi per year
, you are the poster child for extended drain intervals with synthetic oil. Can you get 45kmi on one fill? I doubt it. That will depend on your engine's soot production level. You'll need to go 5-10kmi and have the oil analyzed, then interpolate the results to determine the maximum safe interval. For example, my '87 makes a bit less than 1% soot in 10kmi. That means I should be able to go 25-30kmi before I reach the 3% warning zone (and I think 4% is the absoulte maximum - not sure though). If you do attempt this, you cannot use cheap oil... you're looking at getting Delvac-1, Amsoil, RedLine, or other top-shelf stuff - not Castrol SynTec or Rotella.
Good luck,