start with 4 hours and work up from there. but the best way to determine this is in a prelim experiment. incubate macrophages + LPS at increasing time periods and determine activation by FACS. You can stain for activation markers, CD86 etc. After you harvest the cells at each time period, be sure to stain and fix them using the same stain master solution and isotype controls.

Thank you for the advice Marcella Flores !