This is pretty much the most common question asked on this forum, and since the answer varies from person to person there is no set answer. For a single 8mg dose it usually takes me 48 hours to get a decent high, and 72 hours to get close to full effects. Wait until you are in withdrawals before taking the heroin, since that will indicate that your opiate receptors are no longer filled with Suboxone. If you IV the heroin now, you will probably get a rush, but that's all. Maybe a very short-lived high as well, but you will regret not waiting longer and having wasted your shot.
If you keep going back and forth between Suboxone and heroin then both will become less effective. You won't be able to get as high from the heroin, and you won't get the same withdrawal relief from the Suboxone. Thnk about why you are getting on Suboxone before deciding to try to shoot up heroin when your doctor just dosed you with Suboxone.
Basically I agree with everything that BDP said, and I posted pretty much the same thing as him at the same time as him.
Edit: The other issue is getting back on Suboxone after doing the heroin again. Even if you barely get a high from it, you will still have to wait 18 hours or more until you can safely dose the Suboxone without risking precipitated withdrawals. This is a major pain in the ass if the heroin only stopped withdrawals for a few hours, and then you are stuck withdrawing for the better part of a day before you can dose the Suboxone again. In your case the risk of doing the heroin does not outweigh the reward.