Even with good water levels hard to stretch a trip in NB out that long. Northern rivers ( Kedgwick, Patapedia, Upsalquitch, Nepisiguit) longest trips would be 4/5 days starting way upstream. With this years low water levels by mid summer would be a struggle. In the Miramichi system longest trip would be 5 days and that requires high spring water to start at or close to headwaters.
St Croix is always an option because it is dam controlled. Hardly wilderness but a nice paddle. Could put in as high as the border crossing at Orient/Fosterville and take out at Grand Falls flowage. Easily stretch into 6/7 days. Only three portages, the two dams and Mud Falls. One CLII/III at Little falls, lots of runs and small ledges. We have done it as low as 225cf/s and as high as 2000 on the gauge at Vanceboro.
https://waterdata.usgs.gov/nwis/uv?site_no=01018500Get the rules straight on camping on either US or Canadian side. We put in on Canadian side and camp on Canadian side all the way down.
Littleredcanoe, Mount Carleton PP is always an option. Great hiking.