What do you guys know about the timing accuracy of the watchdog timer?

I would like to use it for wakeup in sleep mode, but I want to sleep longer
than the max ~2 seconds.  I plan to loop through a number of sleeps in my
and wakeup.  What I want to know is how much drift is there in the Watchdog
RC oscillator?  I'm sure there is micro to micro variation that could
possibly be calibrated out using the crystal on power up, but how much in
operation over temperature.

If anyone knows offhand, I'd appreciate the help.

-Erik Reikes