It depends. Some firms pay $5,250 per year or semester, some will pay for the entire thing. It also depends on the degree you are going for. The company I am starting with (mcgladrey) will only pay $5,250 per semester for an MSA (so, part time is almost all of it paid), but will reimburse an entire tuition bill for an MST, even if you take off two whole summers and each bill is $15,000.
I know that if you start with Delloite, & do a full time summer-to-summer program for grad school, they will pay some amount of grad school, and 80% of your salary while you are at school. One of my friends from PWC said they are paying $15,000 for grad school. They wanted her to start the summer before she started, so they would basically pay for the rest in the next summer since each semester is around $15,000. I think it depends on the firm, and their plan for you. They basically tell you when you can go to grad school.
But now, the big 4 wont even hire someone out of undergrad, they go will only hire out of MSA/MST/MBA programs (probably because they can no longer afford to be paying graduate tuition fees). That is new this year. I think EY is taking undergrad, but you have to tell them how you plan on getting your MSA asap. At least this is what I am hearing from my friends who are now seniors in college, in the interview process.