It is not easy to gauge the cooking times if your quantity is less than the original recipe specified. To say that the pan you use is half the size of that specified is not quite enough. A lot depends on the height at which your pan is going to be filled. If the height of the pudding mix in your pan is about the same as for the recipe, expect the cooking time to be roughly the same as per recipe (actually slightly less). If the height of the pudding mix in your pan is half as per original recipe, expect roughly a baking half as much as specified.
In any case, trying to estimate cooking this way is very risky because besides quantity, characteristics of individual ovens do matter a lot. Trial and error should be your best guide. I hope you know how to test a pudding for doneness using a thin skewer or some sort.
The proof of the pudding is in the eating!