you'll probably bill around 2/3 of your time .... not 3/4. so that's 2560 hours. the other thing to keep in mind is that the problem with firm work is that it's unpredictable. there will be time where you're doing nothing and spending your days on discussion boards, and other weeks where you bill 60 hours and work 80.
I think 2/3 sounds a little extreme based on what I've read. Let's split the difference and say 70%.
Which would give us 2688... still a hell of a lot.
However, I do completely agree with you on the unpredictability of it. There will definitly be the occasional insane 80 hour+ week. I think that's to be expected, and perfectly reasonable. It's the idea of doing that EVERY week that terrifies people. I personally have no problem with averaging 65-70 hours a week... but averaging 80 would be a bit much.