703
Context: LaTeX is a typesetting system. When compiling a document, a lot of really in-depth debugging information is printed, which can be borderline incomprehensible to anyone but LaTeX experts. It can also be a visual hindrance when looking for important information like errors.
But of course, how else could you describe yourself as having experience with TeX? /s
I think our TeX savvy lemming here confuses a knowledge level in the expert/consultant sphere with "having experience".
Having worked with LaTeX on and off for 15 years, and on occasions developing TeX macros (ie copy pasting stuff from stackoverflow and shotgun debugging it until it sorta works) and creating various graphics with PGF/TikZ, I would describe myself as having extended experience with the TeX environment. But I still can't tell you exactly what causes \hbox underfull without looking it up... Probably because it's never caused a failure to output my documents.