I work from home, freelance, and often hourly, so often have to commit a certain amount of scheduled time. However, the beauty of GTD is the context system that allows you to work on things based on where you are/have to be. I generally try to just keep track of how long a task might take, to get a clearer picture of my day — but I do schedule in thing that are important and require a solid block of time.
I’m in transit and realise I might be unclear – let me know if that made sense. :)