How much non-thesis work should a GRA do, and how to let PI know if I think it's too much?
Are you being paid as a GRA? Usually your contract should specify how many hours you are being paid for. This is  10 hours a week for some fields (20 for others, as seen in comments), but it can vary. You need to find out what your GRA limit is.

Note that your letter of acceptance -- if it included a GRA package -- may also specify your contractual obligations. If not, check your graduate handbook.

If you have concerns, you should bring it up with your advisor, your Director of Graduate Studies in your department, or your department head (in that order of ascension).