I've heard it take forever to get fired, and they'd much rather encourage you to look for something elsewhere rather than actually fire you. I've heard stories from associates at firms about that one guy who could (or wouldn't) pull their own weight, couldn't write to save their lives, etc. After 3 years of crappy reviews and crappy work, he still wasn't fired; instead he "took another opportunity"
Same here. I knew an attorney who was off her billables for the first nine months (working 40-45 regular hour weeks). She had a "talking to" by senior partners, and then was put on a trial team (where she worked over 80 hours a week). The impression I got was that it's pretty easy to ride through the first year.