When can I stop worrying about spreading a staph infection after a boil in the groin? When can I use my towel normally again without bleach afterward?

Decolonization. Staph aureus may colonize your nose and back of throat and then the skin surface. You should make an attempt to reduce or eliminate this by nasal bactoban and use of a Chlorhexidine containing soap. See your doctor about this. The obvious question, however, is who are you trying to prevent the spread to? If an intimate partner, they are probably already colonized.