Depends on the running time. If it is 3 hours or more like the previous trilogy than that definitely cuts down the number of showings in a busy movie season and would make it nearly impossible to top The Hunger Games for opening weekend.
It would do well for Thursday night-Midnight screenings (if they do Thursdays) but The Hunger Games and TDK were 2h22m and 2h30m so that extra 30-40m or more adds up throughout the day. Both films could fit in one more screening a day than The Hobbit.
At 3 hours and 20m, ROTK did $72 million for their opening weekend. So maybe expect a $100 million opening for The Hobbit and small drops each weekend for a big haul.