It's difficult to give you a clear answer to this as it will vary depending on many factors: power draw (how many electrical features you have running, or the air conditioning/heating), temperature, the engine design itself and whether the engine is sufficiently warmed up or not. On a warm engine with a charged battery we'd say the engine needs to be off for about thirty seconds to justify doing so. The issue is the wear and tear on your starter motor. Modern cars have upgraded starter motors when a start/stop system is fitted, and often have upgraded lubricants or engine components to deal with the increased number of starting cycles without too much wear on components such as the bearings around the crankshaft.