Tough to compute exactly since I lived with an SO, but just counting my expenses I hit around ~25,000/year, which is below the COL figure indicated for my school by about $4k (of course this was several years ago when the COL figure may have been lower--also I didn't buy new textbooks).
This is incredibly person-dependent, though. You can easily spend $40k/year on yourself in SF/NY including rent, going out, travel. You can also e.g. live in gropius and shop only at TJs instead of the prospect whole foods at HLS and cut way under $29k/year, or commute into downtown chicago from a less pricy neighborhood and be closer to $20k than &26k at NU, since living near the law school there is what really stacks up costs.
The reason these figures are useful is that they should be your rough estimation--you might think "oh I'm going to live a super frugal lifestyle and reduce loans," but in law school you'll be way too busy and stressed to always worry about that uber ride or that bar tab or that flight to the friend's wedding or that new computer when your old one was stolen, so its good to build in float.