We started using lights on a timer for e at 18 months, and have just started using them for x at 17 months. It took both of them three days to get it - the first 2 days I was in the room saying "sh, sleepy time" until the light came on and then making a big fuss about it being time to get out of bed, then day 3 I didn't hear them until the light came on and that was that! X has had his light for a week now, and we have had 2 mornings when he woke up early and sing/cried (but quietly, like he didnt expect me to come) until the light came on, a couple of mornings when I heard him about half an hour before the light came on and then he went back to sleep, and a couple of mornings when I haven't heard anything at all until the light came on. E has been using the lights on a timer for 2.5 years now!
I have never tried making it come on later, though - the way I see it is that the price I pay for not hearing a squeak before 7 every week day is that they are allowed to get up at 7 at the weekends too. Now that the girls are older, we have explained to them that the light is permission to get up but that they don't have to if they feel like sleeping later. Over the holidays and at weekends they have decided not to get up even though the light was on a few times.