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There will always be variation, of course, but with multiple answers you should be able to build a picture.
I majored in Computer Science in college, but am working as a Software Engineer at Google. This is a very common path, but is technically different from going into Computer Science itself as a job. That would likely involve a lot more theoretical work than I do.
On a typical day at Google, I start by checking my email (a large part of the company is in California, three hours later than me, and we use email a lot, so there tends to be a large amount that built up over the night after I went home), and then figure out which of my projects to start working on. Sometimes I had a program that needed to run for a few hours execute overnight, and so I base my start on the results that were reported.
The division of Google I'm in has a large, complicated program, which most of my work revolves around. I usually have a handful of different projects involving it at once. Right now, I'm adding a feature for a customer, investigating whether we can change from one source of data to another one, improving some testing tools for the main program on to make better use of Google's infrastructure, and I'm involved in finding places to clean up our code base, in addition to various other small tasks.
On any given day, I pick a project (based on what other people are waiting on, when I get results of old tests, and simply what I feel like I can make progress on) and start working on it. I don't actually spend that much time writing new code in the main program, but I spend lots of time examining it to see how it works and how to add new pieces, and writing tests for the small changes that I do make. I also write a lot of smaller programs, to help me with things like data analysis and making tools for myself and others to use.
I usually work on a couple of my projects at different times during the day (when I get stuck on one, or have a meeting, which usually happens at least once a day, I use those as boundaries which spur me to shift projects).
I work for around 8 hours a day, though it can vary slightly depending on if I'm making a particularly large amount of progress on something and don't want to leave. I don't let myself make a habit of staying longer than 8 hours, though, since some engineers do that and it degrades their quality of life.
While working, I try to stay fairly focused, though have a variety of ways to take small mental breaks, such as quick chats with my friends (online at work, in person at work, or online with people not at work) or websites like Google News. I find that I'm most productive when I give myself brief chances to rest.
So, there you have it. Get into work, check email, cycle among projects (which means writing code and looking at data) until I go home. I greatly enjoy it, though it's not for everyone.
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