Heroin is most stable in mildly acidic conditions, and when kept refridgerated it will keep for a month. But it's not stable in basic conditions, especially hot basic conditions where it breaks down rapidly. (Not recommended however due to bacteria).
So if you were to add dissolve the heroin in water, and a pinch of baking soda, then boil it for a few minutes, most of the heroin would break down, first into 3-MAM and 6-MAM, and these get broken down into morphine. The solution would then contain mostly morphine freebase and baking soda, so you'd have to add citric acid to get it to dissolve into water again.
Also heroin isn't just a way of getting morphine into the brain, it's a way of getting 6-MAM into the brain. It's the 6-MAM that gives heroin it's extra euphoria over morphine. The heroin breaks down in the brain into mostly 6-MAM with a little bit of 3-MAM, this takes a couple minutes. Then it takes an hour for most of the 6-MAM and 3-MAM to get broken down into morphine.
So most of the heroin rush and the best part of the high happens before hardly any of the dose has changed into morphine.
Diacetylmorphine [heroin] breaks down into monoacetylmorphine and the monoacetylmorphine breaks down into morphine.
Diacetylmorphine breaks down twice as quickly in blood as in serum (blood plasma with no red blood cells). And morphine was found from the break down of heroin in the blood, but in serum they found monoacetylmorphine but not morphine.
In answer to your question 24 hours and your heroin will have degraded.