My network operator charges to my account every 28 days rather than every month. How can I change the data usage cycle length under
Settings > Connections > Data Usage accordingly?
You have no option to do so in the stock data usage controls. You will be better off installing a third party app, such as onavo count.
This feature is in Android roadmap. You can just wait or vote for it https://code.google.com/p/android/issues/detail?id=23676