The duties of a TA vary a great deal between different courses and between different professors, and so does the amount of time you have to put in. A few courses require less than ten hours a week. Other courses seem like black holes when it comes to your time - no matter how much time you seem to put in, it's not enough. It is therefore very important that you should not have to spend more than 20 hours per week on the average. You will experience variations in the amount of time you have to spend, but if you are consistently spending more than 20 hours per week on your TA duties, something needs to be changed.
One reason why you could be spending an inordinate amount of time may be that the instructor has given you too much to do. In this case, you need to redistribute the workload between you and the instructor. If you are making up problem sets or computer projects, writing homework solutions, or writing test questions, you may have been given too many responsibilities. Discuss it with the instructor.
Another possibility is that you are not working efficiently. Grading is something that requires practice before you become efficient at it. If you are spending too much time grading, discuss strategies for increasing your grading speed with the instructor. Using simpler grading systems, reducing or eliminating comments on individual assignments, and assigning problems or projects that are easier to grade are possible solutions.
Answering e-mail or questions outside of quiz sections or office hours can also consume a significant amount of time. If you answer e-mail as soon as you receive it, it does serve the students better, but it also interrupts your work and usually sparks an interchange of several messages. Answering course e-mail one time a day can alleviate this problem and encourage students to attend your office hours. The same goes for random office visits. Make it clear to your students that they should not drop by your office without an appointment, and that e-mail is preferable to phone calls.
The other part of managing your time as a TA is making sure you don't get swamped by your own class work. First of all, you should not take too many courses while being a TA. How many is ``too many''? That depends on the courses. For example, taking two heavy courses ( e.g., CSE 501) at the same time would probably be too much. On the other hand, a heavy course could be balanced with a medium-light courses (e.g., CSE 505). Second, try to get ahead on your course work to make time during TA crunch periods, such as after midterms.