Formal:
* Period: "The project is due in the next period."
* Calendar month: "We'll meet again in the next calendar month."
* Lunar month: "The lunar month is about 29.5 days long."
* Solar month: "The solar month is about 30 days long."
* Lunation: "The lunation cycle is a natural phenomenon."
Informal:
* Time: "See you in a time."
* Stretch: "This is a long stretch of work."
* While: "I'll be gone for a while."
More specific:
* Season: "This project will take several seasons to complete."
* Quarter: "The company had its best quarter in years."
* Cycle: "The menstrual cycle is a natural process."
The best word to use depends on the specific meaning you want to convey.