We can't really answer that - it depends on your interests, your skill level, your knowledge, the environments you are used to running C in (I don't write many console apps these days, because it's as quick for me to write it as a GUI app since I'm used to that - but if you only know how to write console apps, that's kinda significant!)
A project that takes me 6 weeks might take you a year, or possibly 6 days - I don't know!
Think about things that you are interested in, that stand a chance of being implemented in C
*, then try to find examples of actual apps - not the source code, just the app - and look at how they "present" to the user. Is that something you feel you could do? Or do you not have a clue how to implement something like that? Then try to "whittle that down" to a manageable list of maybe three to five candidates.
Then go to your teacher and ask him what he thinks - are you being too ambitious? Not ambitious enough?
Only he will know that: he's doing the marking after all!
* project ideas in c language - Google Search[^] may help.
Submit an article or tip