I think our longest so far has been about 10 hours. There is no reason not to have one going for 14 hours, you just need to keep the build environment homogenous (temperature wise), especially when building with heated materials (like filaments).

EDIT: Also, I wouldn't want 14 hours of ABS fumes in my work area, so consider ventilation and/or filtration if you are using material with undesirable fumes.