The answer is a candle. The reasoning behind this is that a candle's life lasts for a certain number of hours as it burns, it serves its purpose by being consumed, and its speed depends on its thickness - thin candles burn quickly while fat ones burn slowly. Additionally, wind can extinguish a candle's flame, making it an enemy.
My life can be measured in hours. I serve by being devoured. Thin, I am quick. Fat, I am slow. Wind is my foe.