Oh boy. Here we go with the macromicromanagement style. Do your sims ever get a moment to themselves?
Of course not. Why would you propose an utterly daft thing like that? Do you even KNOW how the Sim AI works? Macro-micromanagement is what I'm ALL about.
Is there a mod out there that allows you to set the amount of time a sim will take to perform a skilling action, example garden for 30 minutes; fish for 2 hours.
No, and the number of increments involved would rapidly get unwieldy to put in a menu. Not to mention you would incur major transaction cost losses.