When should I switch to 4pin fans?

[gaccm]

I know that if i put a really powerful fan on my mobo's 3pin connecter it could fry it. I'm wondering if anyone knows what the power level is? I know its different for every mobo, but is there any standard, or gray area? If anywhere, where would it be writtin? I've check in my manual and it doesn't say anything about it.

[CrystalMethod]

I won't put any thing on that draws more than 1/4 of and amp on my mother board headers. The mother board headers could probably take more but I don't feel like risking it. You can get adapters that let you connect a 3 pin fan to your standard PSU molex connector.
http://www3.sympatico.ca/keith_lee/connector.jpg

[ButcherUK]

3-4W is the usual quoted amount which correlates with the 0.25A posted above - 0.25*12 = 3W.