I don't think there's a magical number of months that is appropriate for an engagement period. I think what matters more is the quality of that time spent preparing for marriage, rather than the quantity of time. For J and I, we dated in our senior year of college. Which meant, we were mentally checking out of school, we weren't working yet, and we lived on the same campus in adjacent apartments. Even though we dated for a short year, it was a pretty intensive block of time together. However, the number one best thing we did for our engagement period was for him to pack up his bags and move to Taiwan for six months (the longest time the visa situation would allow).
We all behave differently in different contexts. You don't act the same way with your peers as you do with older people or with young kids. Factor into this our cultural environment. One certainly does not act the same in a Chinese context as you would in a Western context unless one did not care for, or are ignorant of, any sort of social conventions. Needless to say Jason was in for a MAJOR shock when he encountered the Chinese me. Somehow, he still managed to get that ring out of his bag and propose as we entered this adventure of our intercultural marriage. During our time in Taiwan, he caught a glimpse of what it meant to be part of a Chinese family, how I behaved when I spoke Chinese and every other aspect of living life in a Chinese context. What was confirming for us that our relationship was headed in the right direction was that he saw a whole new part of me he didn't know before but continued to love and embrace that person. My friend Shannon (American), after spending time with her Chinese fiance in his home town, decided she loved him even more.
With all of my cultural identity issues with the complex background I had, there was no better way to begin my marriage than to have the rock solid belief that my husband has seen all sides of me and can love and appreciate who I was in every context. This anchored me through all kinds of cultural issues after we were married - whether I was rebelling against the Americanized aspects of me or feeling depressed over my lack of engagement in Mandarin, and trying to come to terms with all of this in my faith in Christ.
Next up: differing cultural perceptions of husband/wife roles.