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How much entropy in a key press?

Can anyone tell me how many bits of entropy there are per 7-bit ASCII
character.  More specifically, a program wishes to generate a session
key by prompting the user to type N random key presses.  The characters
entered are hashed down to 128 bits by MD5 for subsequent use as a key.

What should the value of N be, such that the entropy of the user's
string does not unnecessarily exceed the entropy of the hash?