When is the best time to apply for citizenship?

It appears you misunderstand the requirements for eligibility for naturalization. Among the requirements are ones relating to (1) residence and (2) physical presence. Generally, a Permanent Resident must reside in the U.S. for five years before becoming eligible to apply for naturalization (actually, one may apply as early as 90 days before the end of the 5-year period), and one must be physically present in the U.S. more days than be physically outside the U.S. during that five-year period. If a Permanent Resident is outside the U.S. for a continuous period of 180 days or longer, there can become a presumption that he/she abandoned residence in the U.S., but that presumption can be rebutted with evidence that residence in the U.S. was maintained notwithstanding the long absence (a continuous absence of 1 year or longer no longer creates a rebuttable presumption, but instead can cause a legal determination of abandonment of residence). Since you were permitted to re-enter the U.S. after your single trip of six months and 12 days, it appears you were able to rebut the presumption that you had abandoned your residence in the U.S., and so it appears you now may proceed with a naturalization application process. Nonetheless, there really is no substitute for you to consult with an immigration attorney who, after learning all of the relevant information, could advise about eligibilities, options, and strategies to attain your goals, and who then could offer legal representation in the often complex application process.