depending on where youre going, and what youre lifestyle is like, i wouldnt take less than Y100,000 or US$1,000 for the month each, and i would hope that you have access to more via ATM's for just in case.
I was in Tokyo for 3 months, homestay type of deal, so food, rent, and net were taken care of,..... and just from the travel cost of exploring all day everyday and having lunch outside, i burned through Y100,000 a month, and i'm not exactly the shopper frappucino burger tourist trap kind of guy.
i did have a pretty damn good time though
