:) how many days, can I be outside of united states, before apply for a citizenship?

You can ask your question to the Immigration Services. They can and will give you the honest answers.

USCIS Home Page

National Customer Service Center (NCSC) at 1-800-375-5283 (TTY 1-800-767-1833). This toll-free call center has additional information and, during their specified office hours, can connect you to live assistance in English and Spanish. The NCSC will be able to answer most questions - although they cannot provide information about the status of your case over the telephone.