Does working while you are studying affect your GPA? I am studying computer science and I have money to pay for my university study, but I do not have money to spend on entertainment such as going to cinema with my friends in the weekend or upgrading my laptop. I have applied for a job at McDonald's. My shift is 4 days per week with 7 hours per day.

I am scared this would affect my grades. How can I balance my studies with the part-time work I must do in order to improve my living standards or afford a social life?
This study suggests that employment outside the university (especially in excess of 25 hours per week) does have a negligible adverse effect on GPA, and a signigicant adverse effect on the probability of a student's continues enrollment. A second paper (here)  suggests that hours worked do have a significant negative effect on student grades. 


  ...results show that an additional weekly work hour reduces current year GPA
  by about 0.011 points...


A personal anecdote: When I attended the mandatory freshman orientation seminar during my first semester as an undergrad, they recommended no more than fifteen hours per week, suggesting that anything more than that would tend to have a negative impact on our grades. I do not now recall whether they also provided references to back up their claims...

Of course, this is a general guideline, and an individual student may well be able to handle more. I know some students who were somehow able to juggle a full course load, a full-time job, AND major family responsibility, but those paragons are very rare!