I recently picked up some cut up durian at an Asian supermarket, and since pre-packaged fruit normally is a little under ripe, I was wondering how long I should allow the durian to ripen in the bag before Christmas.
It's a matter of taste. Since your durian is already cut, as it continues to ripen (which it will, although much more slowly than uncut fruit) it is also rotting/fermenting (which is desired by many). You don't say where you are from, but durian season is long over in the northern hemisphere. Most durian sold in the US this time of year has been frozen. Previously frozen durian rots more rapidly and ripens less rapidly than fruit that hasn't been frozen, some sources say that previously frozen durian won't ripen at all, only mature (rot).
So the bottom line for you is that you should taste your durian every 12 hours or so and put it in the freezer (or consume it) when it has matured to your liking. The ripening (not rotting) of durian is accelerated by the ethylene it produces. Since sources are not in complete agreement concerning additional ripening of previously frozen durian, keeping the pieces in a bag with ethylene may help. Most of the ethylene is produced by the husk, not the flesh of the fruit. If your fruit still has the husk, it is worthwhile to keep it in a closed bag for that reason. Whether or not the Durian still has the husk, it wouldn't hurt to add an apple to the bag for more ethylene.
If you can, store your durian with an apple in a few separate bags, so that "tasting you go" doesn't release all of the ethylene available to all of the durian.
Some durian lovers go as far as to keep ripe, cut durian on the counter; rotting away (and producing alcohol) for 3-4 weeks. Others prefer durian under-ripe and freshly cut.