According to ScrumMethodology.com, a sprint is 30 days long. However, I can see advantages and disadvantages in having both longer or shorter sprints.
The longer the sprints become, the less agile you really are, especially if you follow the scrum methodology rules that don't let you interrupt or change the sprint in the middle.
The smaller the sprints, the more agile you become; however, it may be more difficult for the development team to get more work done since the product must be in a working version at the end of each sprint.
I feel like the complexity of the current sprints could very well dictate their length. If you're at the phase of the project where you're just resolving minor bugs, then maybe smaller sprints are easier. However, if you're making major changes to the system that require a lot of focus by the development team, then longer sprints may be more appropriate.
Finally, consider that if the sprints need to be longer than 30 days, you may need to check with the development team and ask them if they're designing the system with agile best practices in mind. Many techniques that are popular today, such as using a RESTful architecture, allow for independent deployments of modular components, which helps prevent bugs in other systems and create an environment where deployments can in fact be made more frequently.