Hi, HIV dies when exposed to air, saliva, etc. it is very fragile. there is no set time when it actually occurs but it is rendered inert and is destroyed by the mouth. Tests have been run on hiv spiked saliva and blood in lab dishes. In saliva, it became inert and died, in a pool of whole blood, it multiplied and survived. If your concern is oral sex, that is not a risk
Oh, and to add on; by "immediately" I mean within 1-2 seconds