Ichido(once) is written either in kanji (一度)　or numeral (1度), ichigatsu(january) is also written like 一月 or 1月. You can use both 一二三 and 1,2,3 for ichi, ni, san. There are no regulations, but when I write sentences from up to down, I tend to use kanji(一二三), and when I write from left to right I prefer to using numerals(1,2,3). (Japanese sentences are writen vertically in most cases of literature, and horizontally in cases of mathematic, science, foreign language text.)

MAGO said it all. I would like to add a little about the regulation: I don't know if it still is relevant, but there used to be an agreement about not mixing Kanji and Roman numerals for numbers within a same document. For an example, once you start writing 1, 2. 3... then you were supposed to continue on in roman numerals, and conversely, once you start writing 一、二、三... then you were supposed to continue on in Kanji. In short, if you were not supposed to write 2008年七月10日, instead you were supposed to 2008年7月10日 or 二千八年七月十日. This was how I learned when I was a student back in Japan...