I knew of one legal secretary in downtown Chicago that types 126 wpm error free. I think she's an alien.
Me? 78 wpm 2% error rate and I'm a lifer with carpal tunnel syndrome. Before CTS, it was 95 wpm error free. Now, my hands start to cramp up, I make mistakes, and have to slow down or stop until the numbness goes away.
From novice to 50-60 wpm...tough to say.
If you type every day for a living (8-10 hours like a secretary)...probably 1 year.
Error-free rates by administrative status (for job searches)
40 wpm entry level, receptionist
50-60 average (2+ yrs experience)
60-70 executive secretary
70-80 legal secretary
80+ court reporters; stenographers
Mavis Beacon is a great program but you really have to just type all the time to get faster. It has to be as natural as combing your hair or brushing your teeth.