How much non-thesis work should a GRA do, and how to let PI know if I think it's too much? I am a PhD student, and am being asked to do increasing amounts of non-thesis work. Some of this is general lab paperwork and small extra projects, which I'm happy to do and expect to lead to opportunities to present or publications. However, one project is run on clinical samples as patients present to the hospital, so is very unpredictable and time consuming (about a day's work per sample, split over 2 days). When originally asked to take part, I was told I would have to run samples only when my PI or the main student on the project was unavailable. I am now running almost all the samples, and frequently being phoned or emailed late at night to help with the project, as well as being told to come in to run samples at weekends. This project will be presented by a resident who recruits cases (but I actually collect and process most of the samples) and it is unclear if I will be on the paper. 

How can I gently point out to my PI that this is interfering with my thesis work and my family life, without risking endangering her future reference?
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).