Is the TA expected to solve the homework for the professor or is the professor supposed to provide the solutions to the TA for grading homework?
If the professor wants the grading to be good, then the professor will provide the solutions and guides to the distributions of points. Otherwise, the grades of a student depend on the individual decisions of a TA. Alternatively, the TAs can grade together and create the solution and grading guideline among themselves. The result might or might not conform to the intentions of the professor. Finally, a professor can have a single or a single trusted TA who generates the grading guidelines.
It depends on the professor. Some professors will always generate a solution, and others will have the TA generate the solution. It also probably depends on the experience of the TA. If the TA is going to generate the solution, they have to have some knowledge about the material. A lot of times a TA will be assigned as a "grader" but has never taken the course. Still another option is that a professor will have an assignment that is "close" to an assignment from a previous year. The professor will give the old solution to the TA and ask them to update it for the new assignment.
I think your question really should be "what is a reasonable work load for a TA?". If the grading is taking much longer than 20 hours/week, I would first talk to the professor. They may not understand how long it is taking you, or maybe you are doing more work than they expect. If you can't resolve the issue with the professor, your next step is to escalate to the department head. Some TA's are part of a union.