depends on the playback speed of the finished piece.
24fps on twos will playback at normal speed on a 24 fps sequence, as will 12fps on a 12fps sequence.
Bearing in mind very little plays back at a native 12fps, if you were to add the 12fps material to any other timeline, then you r playback speed will vary depending on the playback speed. So if you put 12fps material in a 24fps sequence, you'll speed everything up to twice normal speed.
Video Editor/Audio Mixer/ Compositor/Motion GFX/Barista
Character Options Ltd
Right, 12 fps is pretty rare. The cool thing about Adobe Premiere is that if I put 12 fps material in a 15 fps sequence or 24 fps sequence, it doesn't stretch the speed. It interpolates it within the time frame dividing the frames. So from sequence to sequence nothing speeds up or slows down. Really nice to have when testing speed rates.