The 5D3 can shoot up to 30 Minutes at a time with the files split into 4GB files (these files playback continuously when linked together in editing). When you get to the end of a card the video stops. When you hit record again it will start recording on the next card. There are no heat issues with the sensor and recording more than 30 minutes unless you are in blistering hot sun (which you still can expect to not have issues). Magic Lantern has a work-around that will start recording again automatically after reaching the 30 Minute mark, but you do lose about a second or so of coverage.
I shoot a lot with my 5D3, always with magic lantern, and often longer than 30 Minutes. So this is from experience =)
2x 5D mkiii, 7D, 60D, T2i, Tokina 11-16mm f/2.8, Bower 14mm f/2.8, Canon 15mm f/2.8 Fisheye, 24mm L f/1.4 ii, Sigma 35mm ART f/1.4, 24-105mm L f/4.0 IS, 50mm f/1.8 ii, 85mm L f/1.2 ii, 100mm L Macro f/2.8 IS, 135mm L f2.0 75-300mm f/4-5.6, 70-200mm L f/2.8, Sigma 70-200mm OS f/2.8. StockhamMedia.com