Two or three years at the very least if it's been in the fridge all its life. Maybe quite a lot longer as its a slow film.
Manufacturers are very conservative in giving expiry dates.....it's just a date when there should be no visible deterioration for all practical purposes, not a date when it becomes unusuable. I'm using Astia dated 2005 from the freezer, and 2-3 y.o. color neg from the fridge with no concerns at all.
The real answer might be to give it a clip test. Of course, if you do not do your own processing this might be difficult. Chances are, the expiry date is conservative and, if stored properly, there should be no worries. I would normally say to give it an extra step of exposure but since this is NOT negative film, doing that might be counterproductive (UNLESS you are processing it as a negative). I use only negative film. - David Lyga