5 minutes. If it takes 5 minutes for 5 workers to assemble 5 lamps, it would take the same amount of time for 1 worker to assemble 1 lamp, because the lamp per worker production is proportional to time. Therefore, it would take the same amount of time for 100 workers to assemble 100 lamps, because 5/5=1/1=100/100. It also takes 5 minutes to finish the work.
