The correct answer is: no article.
The word “rule” is a general term, not a specific one. It does not refer to a particular rule, but to rules in general. Therefore, we do not need to use an article.
If we were to use an article, it would imply that we are referring to a specific rule. For example, we could say “As the rule, we write two excel reports a week.” In this case, we are referring to a specific rule that requires us to write two excel reports a week.
However, in the original sentence, we are not referring to a specific rule. We are simply saying that it is our general rule to write two excel reports a week. Therefore, we do not need to use an article.
Here are some other examples of how we can use the word “rule” without an article:
- As a rule, I try to get to bed early.
- As a general rule, it is not a good idea to eat too much sugar.
- It is a rule of thumb that you should always wear sunscreen when you are outside.