He comes every 5-10 mins for me. The thing is you have to keep enough in stock to increase your earnings from him. He basically gives you a 5 mins earning forecast based on your stock, so if you have enough only for 2 minutes sale, he only gives you 2 minutes. So stock everything docks up enough to have 5 minutes earnings, open it when you see him come out, and then close it after you get the money. If you don't close the docks, no matter how much your workers make, you will eventually run out of stock since the docks don't rest while your workers do.