If hypothetically, Ultherapy were to cause fat loss, when will that start to occur or become apparent?

If done properly, Ultherapy should never cause fat loss. "Hypothetically speaking," if fat loss should occur it would happen around the same time as regular results which is typically 3-6 months after the treatment. Make sure you follow up with your provider for assessment and comparison of your before and after pictures. Best wishes.

Many thanks for your question. Ultherapy does not cause fat loss. Ultherapy uses focused ultrasound energy which tightens deep dermal and connective tissue layers without surgery. The effects of Ultherapy on facial fat has been studied and is not significant. It is a quarter of a teaspoon - basically half the volume of a small marble (about 1.25ml) - if a full face treatment is done - and if all the energy was delivered to the layer which has some fat - which is not the case in the protocols that we use. Thousands of procedures have been one worldwide and the FDA had looked into the issue of the effect of Ultherapy on fat and no warnings have been issued. 
It is important to remember that the ageing process results in facial fat loss and laxity of the skin. Treatment will Ultherapy will result in lifting and tightening and hence the face can look hollow.   
Results from Ultherapy take up to 6 months to develop. You will see the final results at 6 months. It is important to have a follow-up with the person who treated you so that an assessment can be made. Before and after treatment pictures can be compared. 
Best wishes.