It depends. The last few weeks I've been going pretty hard due to my graduate project wrapping up and having a local con right after, so during the last two weeks of the project I did about 10-16 hours of work each day, and during the three days of the con where I ran an artist table I was drawing non-stop from 10am-7pm. On the first day of running the table I'd been drawing so much that my right arm actually starting getting sore, but that was due to fatigue from the day before.
With regards to what Arshes said about crunching for deadlines, sometimes it's just out of your control due to poor management from higher-ups, or laziness/incompetence from the rest of the team. I've read that almost every project has crunch time toward the last few weeks, either because they suddenly realize there is so much work to do and just start getting themselves in gear, or - as mentioned earlier - bad time management.
Just to give an example of this - the project initially was slated to have five short animations complete, split between three in the first half of semester and two in the second half. But because our supervisors never really enforced the mid-semester deadline for the first three, we ended up scrapping the last two and only finishing three for the final. The first animation was in post-production in week 13, when it was supposed to have been in post-production in week 7. So... sometimes things happen that you just can't control.
As for myself, sometimes I find it hard to concentrate and get started for the day. With the internet around and attention-sapping activities like FB, twitter, even checking these forums, I have a tendency to faff about for an hour or two before just saying "fuck Tora do some damn work". But once I get into gear it's easy to maintain the workflow. I also find I concentrate better when streaming, having an audience is a good way to help with keeping attention.