There's a bit of mixed data in the literature. This is an old article from the 70's, the guys were looking at ways of degassing water for use in a Liquid Chromatograph (Dell'Ova V.E. et al (1974) "Ultrasonic Degasser for Use in Liquid Chromatography")
They suggested "it can be seen that a fifteen minute ultrasonic degassing is equivalent to boiling water for six hours"
.. but they used rather an elaborate rig.