Rewriting

"...at what point do you think a new writer should begin to focus on rewriting?"
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Here is the process:
1) Write a first draft in Week #1.
2) Write a second story in Week #2.
3) Write a third story in Week #3.  Polish Story #1 as well.
4) Write a fourth story in Week #4.  Polish Story #2...
and so on. 
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This process is designed to burn the basic story elements into your subconscious at a very deep level indeed.  the week-long gap between 1st draft and rewrite enables you to "cleanse your palate" and  approach the story with fresh eyes.
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But specifically, you can concentrate on re-writing at any time.  Just don't neglect that original writing as well.  You are all autonomous, free, thinking human beings and can take actions contrary to my suggestions any time.  If you felt it was critical, you could take a week and practise re-writing.  But I would seriously suggest writing 1st draft three days a week, and re-writing 2-3 days a week, alternate days.  Let one part of your brain rest while you work other aspects.  This can work spectacularly well, allowing me to produce large amounts of finished work with a minimum of stress.  And excellence comes only from volume work, performed with an eye to constant improvement.  So it is important not to get "stuck" at any level of the process.  For you fiction writers, remember:
1) Write a story a week, or a story every other week.
2) Put them in the mail
3) Keep them in the mail until they sell.
4) Don't re-write (again) except to editorial request.
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A time will come when you feel you have a story that needs deeper re-writing.  If you have confidence that taking the additional time isn't a distraction from your production scedual, then go right ahead.  But be careful--endless re-writing is just procrastination, masquerading as quality control.