Any number like that 2% is pretty silly. Different people have different needs, different current resources, and different comfort zones. The only thing that can really be said about how much to invest is the old rule, "Don't bet more than you can afford to lose", however much or little that may be.
Buffett can get away with large investments because he picks conservative stocks. Investing 20% of your portfolio in Coca-Cola Inc. is far less risky than investing 20% of your portfolio in Google.
