doing other people's work and your own

So, I will be an RA this summer for a public policy prof, working on coding cases and I need some advice. I've been talking to another grad student friend, and she's suggested 20 hrs/week, so that I'm sufficiently engaged with the project, well remunerated, and still with time to work on advancing my own research.

The underlying project is interesting (a judicial behavioral analysis of sexual harassment cases), but I'm basically doing highly trained monkey work--coding cases into variables, for a dataset of about 1,000. Like in two months.

I've already started, reading the coding scheme, background work, and getting set up in the office--I will work from his computer (or a remote VPN client), so that all of the work is saved on the same file. I do not like the idea of commuting, but I wonder if it will make me more efficient, even though I like my ergonomic home work station.

Any advice?

1. Is 20 hours too much?

2. How should I spread out the hours? 4 hours/5 days (the other 5 hours on my own work) or roughly 7 hours for 3 days/week with 2 days + weekends to focus on my own work? I ask, because I have to submit a strict schedule, as he can't access his server when I'm logged on.

3. Although the "work at home or office" question is highly individual, I'm largely agnostic on this--perhaps the thing to do is to just try out the different environments and see which one works best for me. But what have you found to work better for you? I've never had an office (damn underfunded universities), so I am curious. I've always worked from home, and it usually works for me, but lately I wonder if I would be even more productive if I wasn't able to procrastinate by cleaning something.

Thanks for the advice!