I say pick a number that you think no productive employee should exceed. Make that the yearly number, and make sure everyone knows that they will be encouraged to take ALL their days. Make the days expire after a year, so people don't feel the need to "bank" days. Then, everyone knows where they stand in terms of how much vacation they "should' be taking.
For the record, If I were starting a company now, I would make the number of days in the aforementioned system start at 20, and then increase with seniority.