You could probably get away with taking it much sooner than 3 months, probably in the order of 2 weeks, but I would just suggest that instead you take it when you feel it the right time to explore 5-apb again. I would definitely give it at least 2 weeks though. I mean, tolerance will probably go back down in a little over a week, but you don't want to make a compulsive habit out of dosing 5-apb.
Like I said earlier to the OP, I certainly wouldn't take 5-APB in a two week span. You really want to treat this like you'd treat MDMA, IF NOT EVEN MORE CAUTIOUSLY since it is a research chemical and there is no medical testing or short/long term side effects established. You might get away with a decent trip after two weeks but it certainly won't be as magical as the first time. Sounds like a BIG WASTE of money and your own neurotransmitters if you ask me. The only thing that will give you that experience again is time.
There was a time were I was under the misinformed impression that two weeks between doses of anything was enough for me. I was (over)doing empathenogens/psychs (2C-I, methylone, mephedrone, MDAI, 5-MeO-DALT) but none would exactly "hit the spot" so I'd end up doing more. This let to a phase of mild depression, frustration and anxiety over what I thought was cognitive disfunction and compulsive drug seeking behavior (especially downers and I didn't even know why). I don't know if anyone elses chemistry is the similar, but what I'm saying is what comes up must come down. Your brain just doesnt have the physical capacity to experience "heaven" everytime you dose via depleted neurotransmitters and the only thing that will give you an experience like the first is time.
Furthermore, I honestly wouldn't want to take a risk of overdoing 5-APB seeing as we know the consequences of abusing MDMA. Like I said, we don't know the long term effects of these legal substances and they could very well be equal to, if not worse, than their illegal counterparts. Just because it is legal doesn't mean its necessarily safer and okay to abandon responsibility and harm reduction practices.