The requirement for having a long-term contract is determined by the overall scope of service. If your project is a campaign-based project, a long-term commitment is not required.
It depends on the campaign or service provided. In some cases, you will see immediate results; in other cases, results may not be tangible for two to three months.
Most campaigns, on average, take two weeks to get underway. When systems need to be set up or integrated or legal issues need to be addressed campaign deployment can take a month or more.