Ok so here is a rough timeline of a typical IV shot of heroin (of course it can vary from person to person):
onset 0-30 seconds
peak after 15min
plateau from 30min to 2hrs
"afterglow" from 2hrs to 4hrs
end of experience around 4hrs (it will remain in your system a lot longer than that and in people who are physically dependent it will keep you from going into withdrawals for longer than 4 hrs, but 4 hrs is usually around how long the very noticeable effects/high last).
So if you take more heroin before the end of plateau period, it will be fairly close to adding that dose on top of the dose you already took. You may notice less euphoria if you are really high already and your opioid receptors are already saturated, but the risk of overdose is still there.
If you redose during the afterglow period, the risk of overdose will still be higher than if you had taken no previous heroin that day, but it is going to be lower than if you took more heroin while you were experiencing peak/plateau effects from your previous dose. If you wait until after the noticeable effects of the previous dose were gone (over 4 hrs from the injection, depending on the person), your risk of overdose is going to be much lower. I can't guarantee that you could, say, take the maximum amount of heroin you are normally capable of taking without OD'ing and be fine though. I know I've had plenty of times when I was definitely still feeling the effects of IV heroin after 4 hours or more.
The best thing to do is take smaller shots, space them out a bit and just do enough to get you feeling good instead of trying to take the max amount that doesn't cause you to OD. Trying to get as high as possible without ODing can be a very fine line and so many factors can affect the amount which would cause an OD, it fluctuates from day to day, plus the potency of the dope you're getting can fluctuate a lot. There is no precise cut-off time where taking a big dose before that point would cause you to OD while taking it after that point would guarantee you'd be fine. Err on the side of caution and pay attention to what your body is telling you. Remember you can always take more but you can't undo it if you've taken too much.