Lemon juice cannot be substituted for lemon extract because the flavor is not as strong. Most recipes that use lemon extract call for only a teaspoon or two, and a teaspoon of lemon juice will add little to no flavor.
If a substitution is necessary, cooks should substitute lemon zest for lemon extract. When cooks make this substitution they need to use twice as much lemon zest as the called for lemon extract. For example, cooks often use two teaspoons of lemon zest if the recipe calls for one teaspoon of lemon extract. Lemon extract can be made at home by soaking lemon zest in vodka for two weeks, and then removing the zest from the bottle.