In Pokémon GO, Lucky Eggs are rare, so it is best to maximize your XP gains during the 30 minutes that your Lucky Egg is active. The best way to do this is to save up your low-evolution-costing Pokemon (Pidgey, Weedle, Caterpie) and evolve them all after you activate your Lucky Egg.
This calculator determines whether it'd be worth it to use your Lucky Egg based on the amount of Low-evo-costing Pokémon & candies you have.
You can find the calculator here.
Image of calculator:
Some more information from the developer of this application:
"Time estimates are based on a conservative 30 seconds per evolution. This can vary depending on how well the server keeps up. I'll adjust this as the servers start to stabilize." - bchn Reddit user