Antigenic diversity is an important determinant of outcome of infection with HIV. Successive invasion through various modes of HIV transmission may contribute to the increase in the antigenic diversity of HIV. There it is assumed that there exists antigenic diversity threshold level, beyond which the immune system cannot sustain against the HIV which leads to almost complete breakdown of the immune system. At this point, the onset of AIDS symptoms starts with a HIV infected person. This paper narrates a stochastic model used for estimating the expected time to get AIDS through various modes of transmission of HIV. Numerical illustrations are provided using simulation technique to substantiate the results.