not exactly a rigorous study. Also they just broke blinding by letting us know what side was injected w what. Seems like a very pro botox practice too so who knows validity and bias also this http://www.diva-portal.org/smash/get/diva2:882634/FULLTEXT01.pdf
Forehead was the least duration of any of the major areas of face in the below daxi paper (used 32 U for frontalis) - about 4 to 8 weeks less depending upon exactly what metrics are used.