When to write? I never wait for a perfect time to write. The perfect time to write is when and where you feel it, and you know what I'm talking about. It can also vary as to time and place. Writing is like lightening, you never know, it just hits and you must be prepared to capture it in a bottle.
For many who have other occupations and can't afford to stop everything and sit down at any moment and write, I would advise at least having a system in place to capture your thoughts, (feelings and images).
A small digital recorder, pocket sized notebook or a quick note on your phone will help.
Great things have been scratched down on the back of bar napkins since forever, ask any comedian.
If you do have time to sit down and inspire inspiration then good for you. Remember to allow it to flow, don't worry about construction etc at first, this is how you get writer's block. Allow your images out and just have fun, put words to your feelings when you can but don't think about building yet. As the Beatles sang to us, let it be. And it will be.