Project work documentation is a fundamental deliverable of a professional project. How much is enough? Well it depends of course on many factors. However, in my opinion, use cases, class diagrams, process flow diagrams, etc. are not waste of time at all. They have value in various project phases. The only problem with documentation is usually resources.
Some programmers consider documentation is a waste of time. But this is not true. If you view documentation as a display of your design and system rules in an elegant and clear way, you may come to appreciate it more. Also, one needs to put one's self in the position of the people that need to maintain the system. To be thrown 500 files of code and to be asked to fix the problems with them is kind of bad but not uncommon.
I suggest you allocate time in your project and produce the diagrams in cycles. The first would cover the high level aspects of your project and subsequent levels could dwell more on the details.
Use cases in particular can't be deduced easily from code (unlike many other aspects of a system). Make sure you do those.