Well, it depends on a lot of factors, mainly the level of the immunity, when you are diagnosed. This is measured by the CD4 count, (cells that are involved in immunity). The lower the count the less your immunity and the more likely the patient is too get sick.
With the introduction of HAART (Highly Active Anti-Retroviral Therapy). The multiplication of the HI Virus is decreased and chances of survival increase.
Without treatment there are fast progressors and slow why?, is not really known, but strains of the virus are thought to be the factor. For example, HIV2 in West Africa has a very slow progress as compared to HIV1 in Southen Africa