Ultrasonic bath operation time
Beside Jon Custer's comment, to be taken with a grain of salt (as the op mentions a heating unit implemented in the ultrasonic bath), if the manual allows, it depends.

You will recognise that by simple use (even if nothing but water is in the trough), that the temperature of the water will increase simply bu use.  Hence, to keep the results reproducible from one batch, to the next, you power on the bath until this temperature stabilises.  (It is better to briefly switch off the sonification to record the temperature with a thermometer.)

In terms of duration of sonification: while one aims to keep the reaction time under sonification short(er) than by conventional heating (alone), like



(source, doi 10.1016/S0008-6215(00)90155-1)

there are examples, where the sonification reportely has to last much longer than $\pu{90 min}$, for example



(source)

I've seen colleagues run reactions from morning till noon, for the sake of selectivity of the reaction intentionally cooling the bath by intermittent addition of water ice into the trough.

To attenuate the cavatational noise the bath may generate, esp. if your flat bottom vessel hovers with some distance just above the resonators (the hot spots), either consider a different room (or at least, keep the sash closed), or have a "noise-proof shoe box" like



(source)

that you may design by yourself, too, covering your bath durin the operation.  Instead of the foam-lining, a wooden box lined with polysterene is even cheaper, but offers less "relief". It is even better to place the bath (if weight permits) at the same time on a sheet of expanded polysterene that you get "for a smile" at at your departemental chemical shop, or appliance shop. 

But do not forget the ultrasound bath underneath the box / in the other room, especially if your heating unit is used.


(source)

Beside this, you may interested in periodicals like Ultrasonics and Ultrasonics Sonochemistry, or Luche and Capelo-Martínez as secondary references, too.