I took it to mean "I can promise that you'll be hired for at least a month while you work on the project" -- said to a free-lance contractor, for example.
1) "Oh, wow, George! What a lot of work! How long do you think it's going to take me?"
- "I can guarantee that you'll have to work on the project for a month"
2) "How long do you expect my contract to run on this project?"
- "I can guarantee that there is a month's worth of work -- enough work for a month."