How soon after a lumpectomy should chemotherapy start?

A few weeks. Typically chemo can start as soon as one has recovered from her surgery. For a lumpectomy with sentinel node biopsy, it may be as little as 10 days or so. Most patients will start chemo within 3-4 weeks postop, but sometimes there are scheduling delays or delays to place a portacath etc. A reasonable wait has not been shown to decrease survival.
Less than 3 months. There is a good free article that addresses your question: http://www.Ncbi.Nlm.Nih.Gov/pmc/articles/pmc2361615/ this group found no difference in outcome if chemo started as much as five months after surgery. Other studies have shown similar results, though some studies have shown improved outcomes with chemo only three weeks after surgery for breast cancer.