How long can coconut milk last in the fridge?
There are 2 ways of looking at it; how long it will be safe to eat (ie from food poisoning) and how long it will still be appetizing and taste alright (ie not gone rancid).

As you've taken it from the tin it won't get that tinny flavour but as time goes on may absorb other flavours from the fridge so make sure you keep it away from anything that is strongly smelling. Also as around 87% of the coconut milk is fats it will soon go rancid, so keep that in mind, (you'll know when it does by giving it a quick sniff).

In terms of food safety, if it's UHT which most are, then it could probably last for a long time (as long as UHT milk) so about 2 1/2 weeks I'd say. Otherwise if it's not UHT then not as long. I would say about the same time as fresh milk so around a week as it contains sugars and fats and so is a very compelling proposition for any bacteria.

Other sites discussing this: http://community.cookinglight.com/showthread.php?t=14563, http://chowhound.chow.com/topics/291163 and http://forums.lowcarber.org/archive/index.php/t-418419.html.

Hope this helps!