Take it by ear. Once you're in withdrawal to the point where your eyes are teary, sweating and stuff like that then you are good to go, it'll "work" if you take enough, even though your tolerance to the oxys will be jacked up (as you already know). But noone can really tell you that, people's metabolisms are different.
2-4 days sounds about right though. If you're really deadset on doing this, just wait until you couldn't possibly wait any longer before you dose the oxys and see if it's a waste or not. Personally I would stay on 2mg though. What I found was that going from 8mg a day to 2mg a day abruptly and after a few weeks, the 2mg was actually more effective than 8mg. 8mg lessened cravings way more, but also made me really tired and put my metabolism on pause. But 2mg over time actually became a bit euphoric before becoming a habit that i eventually needed just to feel at baseline.
Edit: A better approach might be to use the oxy's to give yourself break from sub like you've been doing and you might even be able to jump down from something like 8->2mg (like youve done) or 2mg->1-1.5 mg much easier than it would be doing a straight taper. That's really a crapshoot though because for me, even if i was deadset on tapering or quitting, sure i might have been able to taper down from 2mg to 1.5 for a few days but eventually i would ramp it right back up and stay right around 2mg. I was also iv'ing.
This might be helpful,
https://www.mdcalc.com/cows-score-opiate-withdrawal it's a tool used by medical staff and the like to put one's opiate withdrawal on a scale that you can go back to instead of subjectively judging your own withdrawal symptoms (which, and it's not like this for everyone, but for me during the WD itself it would be very hard to correctly judge my WD symptoms due to the anxiety induced by forcibly having to withdraw like say for instance i wasn't able to get anything)