How Do I Calculate the Burn Time of a Candle

Before lighting the candle, use a scale to weigh it and find its total weight. Then light and let burn for 3 hours (assuming it's a 3 inch diameter candle). Weigh the candle again. Now you can calculate the burn rate. For example, if you have a 16 oz candle to begin with, and after 3 hours of burning it weighs 15.5 oz, then that means it burned .5 oz in 3 hours. Split that in 3, so in 1 hour .16 oz burned away. Now take your original total weight (16) and divide that by you’re per hour burn rate (.16). This would give you a total burn time of approximately 100 hours.

Note: Another guideline to remember is that a standard 2 ounce sized votive candle mold is rated as a 15 hour votive mold by the manufacturers. Divide by 2 and that would mean that each ounce should burn for about 7.5 hours. This can vary and will greatly depend on your formula, but you can use this as a goal to aim for.
These lines have been taken form http://www.candlecauldron.com/burntimes.html just for information purpose.