From my understanding they heat up at full pelt for a couple of seconds which is when you see the light on the dash. Then they drop to kind of a half setting until engine is at a certain temp. May be warmer weather they are only on for the ten seconds that you mention and then when it's cold stay on for a lot longer. This is controlled by the ECU, sounds to me like it's all working OK. Have you got some kind of problem that was making you think it was the glow plug's?
Jim