I’m going to necro 
this because I have a question to add. 
To the original question, based solely on my observation of how strings are sold—by type (ichi, ni, san) and material (silk, nylon, tetron, etc.) and in different counts (1/3/5), it would seem that it’s normal to replace one string at a time.
My question: when do you replace them? When they break is one obvious answer, but are there other reasons to refresh the strings? I’ve played almost daily for over two months and not a single string has broken. What I do see are indentations or stress marks on the strings and there has been a small, detectable degradation of tone. Should I keep waiting for them to break? Or is 2-3 months time for a fresh set?