On average 2-3 errors. However the real thing we want to look at is what is my chance of guessing right across ALL characters. For 1 it's 95%, for 2 it's 90.2%, and it gets worse from there. The formula for accuracy would be .95^c where c is the number of characters in the password. So the chance of getting EVERY key correct in a 40 character password is < 13% and < 5% for 60 characters.

Right. The comment above is saying even if you are incorrect in 2-5 keystrokes it’s not hard to guess the correct keystrokes if you’re using a sentence style password.

You don’t need to guess every character.

that's pretty high when you can use a computer to run the guesses

What if the password is typed twice? You can easely figure it out then.