Authority in matters of opinion

Hello Rabbi,
I tried to find something between facts and norms. Does the Sanhedrin have authority over facts that concern a fact but are not formulated as a halachic ruling (but only in their opinion that it is appropriate/unappropriate in the eyes of God, but they do not claim that this is a halachic rule learned from poskim/midrash/the Central Board of Jewish Studies/Tekna, etc.)?
For example, if the Sanhedrin determines that in our day it is not appropriate to pray for more than 20 minutes a day (just an example), on the one hand this seems to be a factual question and then it is no longer within its authority, on the other hand if it is not appropriate then I would probably refrain from eating schnitzel and perhaps this is a mandatory norm.
To illustrate, if someone proves from the midrashim in the Gemara that it is not appropriate to do such and such (even an existential mitzvah like praying or putting on tefillin for more than a few minutes), but it is not worded exactly according to the law (forbidden!), what is the status of this?
thanks!