The guideline to obtain a conclusive negative test has been 3 months since 2004.
Here it is:
It is true that in some people it takes 3 months for detectable antibody to develop. But it's quite rare. When the risk is especially high -- for example, after unprotected vaginal or anal sex with a partner who is known to have HIV -- it is wise to test at 3 months or later. For the average level of risk, or after low level exposures, 6-8 weeks usually is OK. The reason official advice usually says 3 months is that the people who write that advice cannot know how high the risk is for any particular person. So they take a conservative position, in order to avoid giving people false assurance.