When: As often as possible. My full-time job schedule varies in hours and days of the week so it's perfect for me to write at various times. I have no set schedule for writing. If I found myself not writing, I would create a set schedule.
Where: Mostly in my apartment. I live in a 500 sq. ft. studio (perfect) with my iMac on a library table and a recliner in front of it. I type from my recliner with a wireless keyboard.
If I'm stuck: and I want/need to write, the most helpful thing for me is to go somewhere that's similar to the surroundings in the scene that I'm currently writing. I've sat on benches or planters in downtown Minneapolis with people scurrying all around me. I've gone to the nature conservatory and written there. Malls, parks, rivers, parking lots, neighborhood streets where I sat myself down under a tree between the sidewalk and the road, etc., I've even gone to IKEA when I needed to feel like I was in a character's house, whether it was the kitchen, bedroom, living room, etc.,
This really, really works for me. It helps me be more aware of my senses and reminds me to keep it real. It helps me to add appropriate sights, smells, noises, etc., Sometimes just the traveling to get to these places clears my mind enough. When I need absolute quiet, or when I can't find similar surroundings to write in, I go to the library and use their quiet rooms.
If I don't feel like it: I usually write anyway and keep the discipline. If I truly just don't feel like writing, I don't. I know I'm disciplined enough to see through my own excuses and work in spite of them. So, when I really can't bring myself to write that day, I don't force it.