In every movie that has a "suddenly-it's-a-few-hours-later" shot, if they're burning big thick candles, the fact that time elapsed is usually shown by the fact that the candle is suddenly reduced in height. However, I know this is another bit of movie magic because all the big thick candles I've ever had always burn right down the center and the edges usually retain their height, so I end up with a candle that hollows itself out as it burns and I eventually have to slice off some of the excess wax with an X-Acto knife in order to get to the wick.

Although if they actually have special movie candles that burn evenly all the way down, I think they could make a decent amount of money from selling them to the general public. I'm sure some candle chain could partner with the studios to offer us poor saps an "Official Movie Candle(TM)." And I'm sure people would buy it too.