I write when I can! I know that sounds simplistic but it’s true. Since having a baby (and with another one on the way!) I grab any opportunity I get.
Generally it depends on what stage of the process of writing a novel I’m at. If I’m still in the first draft, just-get-it-written stage, I usually write for a few hours every night after work. Then I do at least 10 hours over the weekend, baby permitting.
If I’m editing, I tend to do shorter stints more often, because I become word blind to my mistakes. I’ll usually edit in 25 minute chunks, take a break for 5 or 10 minutes, and then start the whole process again.
I also try to write wherever I can if the mood strikes me. I write using my Galaxy Tab, my phone, notebooks… Everything! Train journeys are particularly productive for me, though I don’t have much cause to be on trains these days.