A day in the life of a Class Rep... is just like any other undergraduate.
You go to class as much as you should and you work as has much as you want, your extra requirement is to listen. Listen to the feedback from your class and to any other personal questions they may have. Pay close attention to the misinformation in your course and to the organisation problems, so you can later address them and find a solution.
For me, a Class Rep is the person who as the ability to enlighten the class when everything is confusing.
There are only two days you will not live the life of a normal undergraduate. This only happens once per semester and takes 1 hour. On these days, you have the Student-Staff Liaison Meeting where you join the other Class Reps to discuss yours and other years problems.
In the 1 hour meeting you find how the University is listening to your particular concern and the solution they can offer. You have insights on how lecturers prepare your course work, and the reason examinations are so important. The problems are often a variation of issues that older Class Reps raised. However, every year the university tries to improve their curriculum and solve the mentioned problems. Your positive feedback is the proof the University solved it.
There are times, normally when the Class Rep meeting is not scheduled to happen for a few weeks, where the best solution is to schedule a meeting with your lecturer to resolve the situation. These meetings normally involve re-evaluation of grades and solution for re-sits where your head of department is a mediator and both the class, and the lecturer agree on a solution. For other problems like tutorial solutions, sending an email to the lecturer settles it.
A Class Rep connects the class, either connecting everyone in same group chat or making everyone aware of the social nights. For some courses, you can mention events and projects the University are doing that relate to your course.
In reality, there isn't any normal day even for an undergraduate. At least being a Class Rep gives you the chance to fill in the gaps that the course or the lecturer are causing. The department needs your course feedback in order to understand how deep the gap is and how much help you need to solve it. The role gives you the chance to communicate with everyone from student to Head of Department, and even learn from the upper undergraduate's experience of the course. But do not be scared as a Class Rep you have the chance to resolve it.