Ok, I just arrived back from the Philippines a married man. I was only there for 2 weeks but luckily everything went ok and we managed to get married without any problems. We went to the office one day to give the details and sign the forms, and then went back 10 days or so later for the ceremony.
In my experience this was easily achieveable in 2 weeks, but I still think it's best to follow other peoples advice and allow more time incase something goes wrong.
Once again my main advice is to get your fiancee to get the CNI for you. This will save your precious time when you are there.