Revision is a personal thing. Some writers tend to revise once the first draft is done. That's Writing 101, I guess. I revise every day. The moment I open the manuscript I immediately go to the previous day's work and edit and revise. I do this every day while writing. Takes me about an hour. Once the draft is finished, I read it in its entirety and make notes as I go along, then start the bulk revision. Once that's done, I close the manuscript for a week or two and when I come back to it, revise once again through fresh eyes and a clear mind.