How long can you take tylenol (acetaminophen) pm (i.E. How many consecutive nights) for it to remain effective and safe?

Efficacy not good . In spite of the fact that it seems that many people are able take Benadryl for days on end and it still works, i really don't see any necessity of adding in the tylenol (acetaminophen). And studies on Benadryl alone, most people lose its efficacy in lab studies after about three days of use. Then they have to wait several days before it will actually work again. Add to that the impairment of driving in the morning that has been documented in studies and the potential for weight gain and Benadryl and you really do not have a very ideal sleep medication there.
Very safe. The Benadryl in the tylenol (acetaminophen) pm is what makes you sleepy. It is non habit forming so it is safe to take for a week without a problem, i would not suggest taking it for months at a time and don't drink alcohol with tylenol (acetaminophen) as it can hurt your liver. You should also consider no caffeine for 8 hours before bed. Slighter cooler temp in your bedroom. No vigorous exercise for 4 hours before bed.
It stops working. Everybody is different but i bet you if you are taking it daily, probably within 3-8 weeks it won't do the same for you.