Chinese numerals or Arabic numerals? In formal writting in Chinese, should we write


  事情已经过去了一万二千三百四十五天。


or


  事情已经过去了12345天。


or


  事情已经过去了12,345天。


Which one is considered more standard?
In formal writing in Chinese, should we write 事情已经过去了一万二千三百四十五天。


Only when you are writing in vertical writing format -- from top to bottom, right to left in columns



As for horizontal writing format like English, when we see sentence like "My phone number is 001-2000-110" we would read it as "my phone number is zero zero one- two zero zero zero- one one zero" but we would never write it that way. The same is true in Chinese if you write horizontally