I suspect that, like defrosting in a microwave, it will be down to trial and error.
I've just bought some short-dated salt fish patties and put them in the freezer. (The label says they can be frozen but gives no guidance on defrosting them) . To defrost them I'll put one in the microwave for 70 seconds (simply because past experience has taught me that's the minimum time that most things take to defrost). Then I'll test it with the back of my hand, to see if it still seems to be chilled. If it is, I'll give it a bit more time in the microwave. I'll keep doing that until it seems to be about room temperature. Then I'll stick a skewer into it to see if there's any 'crunchiness' left. If in any doubt, I'll give it just a little longer in the microwave.
I think that you'll almost certainly have to adopt a similar technique for defrosting with your fan oven. Nobody, either in a manufacturer's handbook or on the web, will want to offer a precise time, for fear of leaving you with a pasty which is only partially defrosted.
Chris