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?