How much entropy is generated by 'random' human keystrokes?
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.