Contracts are usually month to month. In other words, the minimum purchase is one month (i.e., four weeks). In some instances, a 2-week advertising period is permitted. You can advertise for as many weeks as you want, depending on availability (all the way up to 52 weeks).
The minimum purchase requirement is listed in the Rates section of each ad format's page. For most ad formats, the minimum purchase requirement is $5,000 per ad format per market or higher (depending on ad format and/or market), which may be spread over displays and periods. For certain ad formats, a minimum quantity of ad displays may be required instead. Minimum requirements may be reduced in certain cases.