The short answer to this...
NOT AT ALL!
In my regular life, I am a major multi-tasker. I learned how to multi-task as a buyer. I mean, when you need to juggle multi-million dollar buys, with merchandising, marketing and still meet a personal sales quota weekly - you had better get good at doing many things at once. When I switched careers and started practicing school psychology, I found that my skills at multi-tasking enabled me to do a lot more in that setting as well. I am able to coordinate projects, keep track of multiple cases, conduct professional training throughout the district and county, and handle both my counseling and assessment caseloads with ease. And yes, juggling the many hats I wear absolutely takes some serious multi-tasking.
However, things are very different with my creative endeavors. In the beginning, I tried to multi-task with my writing, focusing on a WiP and a different project for revising at the same time.
Um...BAD IDEA! I just can't juggle multiple writing projects - not even projects in different genres. I do a lot better creatively when I focus on one thing at a time.
Currently, this means finishing edits on TRANSCEND so I can get ready to query it. Once that is complete, I have a NF proposal to finish for my editor, and another WiP just DYING to be written - plenty of things to keep me busy through the end of the year.
What about you guys? Can you handle many projects simultaneously? Or are you better focusing your efforts on one thing at time?