Where it fits in your routine
First read the product's intended use because lactic acid may be an exfoliating ingredient or a pH adjuster and an ingredient listing alone does not define a routine.
If it is an exfoliating product, check whether it is rinse-off or leave-on. Follow that product's contact time rather than borrowing instructions from another acid.
You do not need an extra exfoliation step just because a familiar moisturizer lists lactic acid among its ingredients.