I think these are empirically determined parameters - it means that you will have to perform experiments with various duration of sonication and to follow the amount of extracted product (for example, by some chromatographic, spectroscopic or gravimetric method). Optimal time for extraction should be the one, after which you can not extract any more significant amount of the substance you need. Time for extraction strongly depends on the type of your herbs, theirs state (fresh or dried), their dispersity and the solvent system used.

I think these are empirically determined parameters - it means that you will have to perform experiments with various duration of sonication and to follow the amount of extracted product (for example, by some chromatographic, spectroscopic or gravimetric method). Optimal time for extraction should be the one, after which you can not extract any more significant amount of the substance you need. Time for extraction strongly depends on the type of your herbs, theirs state (fresh or dried), their dispersity and the solvent system used.

Rightly explained by George. It can't be generalized. The duration for ultrasonication and its speed depends on type and status of plant material, extractant, etc and should be optimized experimentally.

Answered completely without any doubt by Dr. Georgi and Dr. Yogesh

Indeed, as answered above, you have to assess this. To give you some idea: for glucosinolate extractions we sonicate twice, the second time adds only a few percents (~5-10) to the total, but does reduce the varation between samples somewhat.