For an interview, "medium" is not clearly understood. I suggest you put a number whether it's days, weeks, years etc. Example: I have 5 years experience in C programming language.

As a matter of language, it isn't correct in grammar or word choice. This would be better: "I am an intermediate level C programmer." This is not a good interview answer, though, because there are no established standards and "intermediate level" doesn't tell the interviewer anything useful. As an interview answer I like Edd's answer--"I have five years experience programming in C." Even better would be a short answer that tells the interviewer HOW you used C and WHAT you accomplished with it; for example, "I used C to write the lexical parser for my company's internal scripting language..." "I used C in an embedded system when I rescued a project by taking over for an engineer who left suddenly..."