There's a comminution limit for all materials. Below a certain size (dependent on the chemistry) they will either deform plastically or fracture and recombine due to van der Waals attraction and solid-solid diffusion. See:
Dispersion and nanotechnology https://tinyurl.com/y2wfzed7
Adhesion and cohesion http://tinyurl.com/zwb2wlh
Controlling powder bulk density by optimizing particle size and shape distribution https://www.brainshark.com/malvern/vu?pi=271602698&b=1&tx=120486&c1=22
Most of the ultrasound energy in a volatile liquid such as ethanol is converted into boiling the liquid, thermal and heat artifacts, sound, and light (sonoluminescence). See:
Ultrasound, cavitation, and the singing kettle http://tinyurl.com/olueohz
The amount of energy introduced into a system is dependent on the size and shape of the vessel, type of ultrasound probe, temperature, dissolved gas content. In general water is the best system into which ultrasound can be introduced. See the above webinar.
I suspect that whatever you do, even with an 800 W Soniprobe, will not reduce the size significantly. However, good luck and please prove me wrong. I learn more when I'm wrong than when I'm right...
It is difficult to answer the question in terms of precisely how much time will be required to make a nanofluid but rather it is important to form a stable nanofluid on sonication. So the answer would be the amount of time required for formation of stable nanofluid. It means the nanofluid should give excellent service in the intended purpose provided it is stable or in other words the mixture of GO-SiO2 remain in the body of the solvent by getting themselves distributed uniformally and forms cohesive colloidal dispersion.