I very often see transcriptions of French speech abbreviate "tu as" to "t'as" and "tu aimes" to "t'aimes" and similar such contractions.

I understand this is not formally correct but is generally how people talk when speaking quickly.

However, I'm curious, in writing, to what extent is this form casual? I'm hoping someone who's fully fluent in both English and French can make an analogy to something in English.

In writing, when is it appropriate to use *t'as* as opposed to writing it out fully?

I gather it's normal in chat conversations but wrong for a job application. But surely there are many levels in-between...?