It depends on what kind of work you are doing. Right now, I'm working on a petitioning campaign in California. The job ends in two weeks. I spend my days approaching people on the street to gain support for the specific initiative we're trying to get on the ballot. I've done similar work as a fundraiser. The day starts around 9 or 10 in a busy part of town and you spend the day talking to people. These are entry level activist jobs. From here you can go on to supervise and manage campaigns, work for the specific companies, etc. It can be challenging, but working for something I believe in always leaves me with a smile on my face by the end of the day.
As the other people said, it depends on what type of activism you do. I've worked on behalf of communities, families, and kids around the US for 15 years at various nonprofit organizations. I read research, design and implement programs, partner with people at other nonprofit and government agencies to make change in how programs operate and who they serve, do data analysis, and do a lot of writing! I speak at conferences and go to a lot of meetings. Every day is different. It's great working with other people who want to make a difference. You won't get rich, but depending on what path you take in this field, you can earn a very fair iving.
First off depending on the nature of what you are trying to advocate or defend. Either way lots of work with no pay. That is the beginning. You earn your way in this field of lobbist, you need a degree in political history or law or some sort of business field, you need credentials to get your foot in the door for people to take you seriously. Unless you are a founder of a group of people for change. Many legal forms, terms, a must know law and amendments to go into this field. A broad knowledge of whatever field you choose as an activist. You will stay busy, much computer work, history, proving theory, or whatever, activist have tough jobs.