As TheSirDuke says, it can really vary - here's my own take on a fairly common situation: working on-site as a UX Designer amongst a team of designers and developers (who are either subcontracted or part of the client's internal team):
* 8am - get in early to beat peak-hour traffic and get myself organised before the day begins: checking emails, adjusting the day's task list, preparing for morning meetings/sessions, or finishing off things I should have done the day before.
* 9am - I like to socialise a little with co-workers as they arrive, so we start the day in a friendly and collaborative manner. This may be a simple good morning and a quick chat to catch up on things since we last spoke (sometimes a few days prior if I'm only onsite part-time)
* 10am. We'll also have a quick 5-minute 'stand up' meeting once everyone has arrived (both physically and mentally) so we all know what we've completed, what we're working on today, and any roadblocks to getting things done.
* 10:15am - I feel more energetic in the mornings, so I book in my most ambitious tasks then, if I can - heading out to conduct the day's interviews or test sessions, sitting down to generate ideas, getting my head around new concepts or setting up new projects. In the mornings I also have bite-sized, practical chats with specific people to plan, steer or work on the days/weeks tasks.
* 11am - join a colleague on their trip to the kitchen for a morning tea/coffee. Chat about ideas, what they're up to, and inspiring stuff we've come across.
* 11.15am - I use the late morning period before lunch to finish preparing for meetings after lunch
* 12.30pm - if I'm organised I'll have brought a salad made from leftovers and eat with one of the teams from around the business. Otherwise I'll nab someone and head out to get a sandwich (or maybe some Indian takeaway, yum!)
* 1pm - People tend to be a little sleepy after lunch, so that's when I get any external people or resources lined up - such as test session or enquiry participants, data from various sources or providers, check the backgrounds of prospective hires or subcontractors, etc.
* 2pm - There's always a pile of my own tasks I need to get through, and I often spend the afternoon crunching through this, or locking myself in a meeting room with a designer and/or developer to collaboratively create the next iteration of a product prototype or troubleshoot some issues with our various perspectives.
* 4pm - By late afternoon I'm beginning to feel a bit lethargic and unable to focus, so I'll either take a break and make up with more productive time later in the evening, or head out for a quick lap of the block to get some fresh air before getting stuck back into things.
* 5pm - By now I'll have finished all the collaborative tasks for the day and some of the team start to head home. Before they go I try and touch base with exactly where they got to so I can plan for the following day.
* 6pm - If the developers stick around later I'll spend some time swapping links and technical references, tinkering with some new system or infrastructure ideas or degenerating the situation into a Nerf battle. People in senior positions such as digital strategists, digital producers or heads of department will often still be around too (trying to chip away a bit more at their massive task lists), and it's a chance to have a quick discussion with them about things too.
* 6.30pm - The worst of afternoon peak hour is over and I'll venture out and make my way home again.
* 8pm - after dinner I'll read some books, check some RSS or blog posts or links, do some sketching or mind-mapping, or perhaps write up some notes or a blog post of my own. I try not to obligate this time, but leave it open so I can explore some things I'm passionate about.
There's also some fantastic insights into the work styles, tools, techniques and processes of ten of the world's leading UX professionals in the Everyday UX ebook Well worth the read!