Is it possible to use 'than' instead of 'as' here?
Yes, they are equally grammatically correct. However, as and than have completely different meanings in this context.
It would be spoken more as
This construction talks about the correct way to phrase something; which words to choose. It can also be said as:
- It would be spoken more like
- It would be spoken more along the lines of
- It would be spoken more in the manner of
Here, as creates an equivalency relation. What comes after is more or less the same as what comes before; the correct way to say it, in this case. The context tells us in this case that the sentence is comparing ways of speaking; as can be used to equate any two comparable things or qualities.
It would be spoken more than
This construction talks about which of the two is said more often. It means the same as:
- It would be spoken more often than
- It would be spoken more commonly than
- It would be spoken with greater frequency than
Using than creates a frequency relation in this case. It compares the incidence rates of the subject and object. However, than does not always talk about frequency; it can be used for any general comparison of amounts or quantities. In this case, we infer that it means more often than from the context.
Is there certain time I should only use 'as', not 'than'?
This is extremely difficult to answer, because it depends almost entirely on what you want to say and how you want to say it. Notice how in your example the meaning changes entirely (word choice versus usage frequency) as the result of such a minor edit? This happens because the context is just that important.
As and than have different meanings when considered as single words. But we never use them in isolation; they simply don't function as interjections or ejaculations. And it's both easy and common to construct phrases which might appear similar to a non-native speaker but have quite different meanings, or vice versa.
However, in general, you might consider these rules of thumb:
- As creates a relationship that conveys more or less equal or equivalent
- Than creates a relationship that compares magnitudes or amounts which are usually different; greater than or less than
These are absolutely not hard and fast rules, merely a general approximation of the most common usages.