How much entropy is generated by 'random' human keystrokes? I'm wondering about using human input as an entropy generator. We all know humans are terrible at making up random numbers or keys or passwords. But what if we let a user 'randomly' type in some garbage. Just smashing the keyboard, so to speak. 

How much randomness does that give, or rather, how much bits of entropy would it generate?

Obviously, way less than 2log(number of keys on keyboard), but I'd say 'a few' bits per keystroke feels about right? After one keypress, the next will be either a key close by (with another finger of the same hand) or one on the opposite side of the keyboard (with the other hand). So I would roughly guesstimate that at somewhere between 2 and 3 bits per stroke.

Any insights?
http://csrc.nist.gov/publications/nistpubs/800-63/SP800-63V1_0_2.pdf

This publication from NIST tries to estimate the entropie of user choosen passwords. I know, it is not exactly what you requested, but i think it could be a good indication for the entropy of "random" keystrokes.

Especially Appendix A2 is relevant. The assume the following:

The first character is taken to be 4 bits of entropy
The next 7 characters are 2 bits per character
For characters 9 - 20 the entropy is taken to be 1.5 bits per character
For characters 21 and above there is 1 bit per character

There is a "bonus" of 6 bits of entropy is both upper case and non-alphabetic characters are used

For more detail read the Appendix A.