Zero. Cologne has a very agressive policy on charging for street-parking - I think the metered parking around the hotel runs something between 8am and midnight, and it IS enforced during these hours. And it is something like 2 Euro per hour with a four hour maximum.

If you can stand to park next door (literally!) it saves you 10€ a day (no in and out, though)...the garage is called "Rhein Triadem", and the entrance is right across the steet from the Marriott garage...you drive down the same street and make a left for the Marriott's garage or a right 20m earlier for the Rhein Triadem.

Greetings - Dirk