When does the Eiffel tower sparkle and sign really bright? How often does the Eiffel Tower's elevator break?
Besides the Trocadero (free), another great place to see the light show is from the top of the Arc de Triomphe (covered by a Museum Pass).
The light show is most visible after dark. From your other threads it will be getting dark quite late. It happens hourly. When it happens you will notice, but you see a lot more looking at the Tower than from looking out of the Tower. How often do elevators in major buildings in SD break? About the same frequency - not very often. Not much of a worry, unless you like worrying or are actually scared of elevators. If it is the latter, they are very reliable and not small - they will be full of people.
A good place to watch the light show is from the Tocadero, across the river from the ET. It happens at the top of the hour and lasts about 5 minutes, beginning at dusk.