How long can a person who has been diagnosis with HIV virus can sustain their health before getting on meds?

Not smart. Starting meds asap help tremendously to maintain health and decrease risk for complications. Please look at magic johnson!
Variable. Hiv infection can remain asymptotic for many years or could be symptomatic in few years or during acute infection. Decision to start HIV medication is multifactorial. If you start HIV medication early it may prevent HIV associated cancer and or other complications. So it is not important how long you can wait. Better to start early.