As a lab assistant, how many hours per week am I expected to be helping out?
In graduate school, this would be dictated by your GRA (graduate research assistant) contract, which would specify a minimum number of hours you would be expected to work on research each week.

As an undergraduate research assistant, if you're getting paid OR if you're receiving credit for your work, I would expect this to be dictated by your contract or a syllabus (respectively). If neither of those conditions are true, then this is a matter to take up with the professor you work for. Consider telling them that you would like to cut back on your responsibilities to pursue other interests, but know that they might counter with telling you that you're no longer needed. Negotiate carefully and respectfully.

For the record, 14 hours doesn't seem too extreme, even for an undergraduate research assistant. And if you're being paid, whether or not you can be asked to grade depends largely on the terms of your contract, though it doesn't seem unreasonable. In any event: negotiate respectfully with your boss, just as you would do in any other job.