Depending on how much visibility you need on ongoing costs, you will have to decide whether to use 1 or 2 week development iterations.

A 1-week Sprint is for small projects (less than 2 months) or if constant visibility into costs is an important factor, as it gives better feedback to the Product Owner.
Note: This can be at the cost of increased overheads.

A 2-week Sprint is the most common and allows a reasonable amount of work to be done for each release, while minimising Project Management overheads.

A 4-week Sprint is a smell.

It is important to note that as more project management overheads are added, these will have to be paid for.

"The more you want to see how much you're spending, the more you'll spend". - Ulysses Maclaren

It's important to find the right balance for you.

Internal Projects

For internal software development projects, since there is less cost involved in project management overhead, 1 week Sprints are recommended.

If there has been less than 5 man-days of effort during the Sprint, however, (e.g. due to leave or interruptions), keep adding 1 week to the Sprint until you have at least 5 man days of effort worked.