This depends entirely upon what your mentor wants you to accomplish in 5 months, how much resources she can spare for you, and what is your level of enthusiasm for learning and achieving something.
Jurkat cell line has been around since before I was born (which was not last year), so while you can do things like optimal seeding density for months on end, if I were in your place, I would say what is the point. But that is me, I was more ambitious even as a undergraduate student.
Tell you mentor that you will give it your all, that you are willing to learn and work hard for it, and ask for a worthwhile project that only she can think of in the context of her resources and research direction.
Sorry, no definitive help, but you may thank me later.
So. Now that you have your first ever question on bioforum answered (or not), mail yourself your username and password so you don't forget them, and then come back soon to update us on how it all worked out. That's how you build Karma in science.