Chinese numerals or Arabic numerals?
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