If Michael afton is the one to have the nightmares, how long has he had them

If Michael afton is to be the test subject to the hallucination gas since 1983, how long he's he been there, is it until his death 1993 or William just stopped at one point, cuz if he had them from 83 to 93 I am 100% sure that the hallucination gas got stuck on his body and bro start see shit