Whenever you feel like its time to write.
Never let Dramatica (or any other writing tool) impede your desire/need for creative expression. When the urge comes, go for it. There will come a point when the flow runs out or feels "stopped-up." That's the time to go (back) to Dramatica and use it to look at what you have.
Some writers require structure to inspire creativity. Dramatica can help these writers develop a workable structure up front to spark their creativity.
Other writers require total freedom to inspire creativity. These writers get to a point when the creativity tapers off and they have to make sense of what their creativity has produced. That's the time for these writers to use Dramatica.
Whichever writer you are, write when the urge strikes you and use Dramatica to make sense of what you write.