I think you are probably nearing the end of your physical withdrawals, this being the end of the 3rd/beginning of the 4th day...and congrats on getting that far, as that is no small feat! I have struggled with IV dope addiction for 17 years (I'm 32) and must admit, I don't think I've gone longer than 12 hours with no dope, I got high on the way to enroll in the methadone clinic for chrissake lol...but I digress.
Staying clean & sober is going to take a lot of work, such as figuring out and getting to the root of the problems that drove you to self medicate with the pills. I've been (mostly) sober for beginning my 5th year now, and I can tell you, I still struggle with some issues on occasion (using dreams, urge to use, thinking about that pretty red trickle flowering....I digress yet again) I can say, for me at least, as time progresses the occurrence of these using ideations and tempting thoughts are fewer and farther and father in between, and I find it progressively easier to shrug it off so to speak, and change the thoughts to more productive ideas- but the occasional anxiety inducing craving (or is it anxiety induced? hmmm, thats something to ponder

) still rears its ugly head, even with the help of MMT...
And I am sorry I can't tell you how long your anxiety will last, this is really highly subjective and different for everyone. I feel anxious every time I drive down the road that runs thru the dope spot, which is sometimes unavoidable, as it was just as recently as earlier today....best wishes and keep us posted!