How to sell logo design?
As has been pointed out you will have to sell the full rights of the logo to the company that buys it – any customers will not stand for a commission payment every month. Instead of working on just a logo you should be looking at the full corporate identity, applying the logo and other information to letterheads, business cards, vans (if applicable) every situation that a client might use a logo for. Clearly I cannot tell how much or what you are offering, however, 5-15 days is a long time to create a simple design, small to medium sized companies will not be prepared to spend a huge amount of money for their logo design.