Naples luggage storage

Hi, I will be visiting Naples this year in June. From Rome, I will be taking a train down to Naples from where I will take a Circumvesuviana train to sorrento for 2 days. Following this, I will take a ferry or hydrophoil to Capri and then back to Naples. I had a question regarding luggage storage. How long can the Naples luggage storage facility store our luggage, perhaps 2 suitcases? Will it be safe to leave the luggage in the Naples luggage storage for 3 days. How much will it cost? Thanks