How long does it take for a medicine to build up in your system and start working after being off of it for 3months, the med is haldol (haloperidol)?

Just as long as 1st: When you are off meds for over a month, I would expect it to take as long to work as when you first started. You may have the advantage of knowing the appropriate dosage and may shorten titration time. Haldol (haloperidol) generally works fast for agitation(a few days) but take longer for paranoia and voices. Keep your prescriber informed to know whether you are getting better or side effects