How much time should I spend on reviewing a paper? I never tried to avoid becoming a sub-reviewer, and still, this question made me wonder. How much time should I, as a PhD student, spend on a review of a paper that my advisor gave me to do? Does the answer depend on whether I'm getting credit for the review or not, that is whether I'm a sub-reviewer or not?
You should do slightly more reviewing work for the community than the community does for you.  So, as a rough estimate, assuming every paper you write and submit needs three reviews and your average paper has three authors, you should review slightly more papers than you submit.

There is no way to predict in the abstract how long a paper takes to review; that varies from field to field, subfield to subfield, and even paper to paper.  Answering a similar question in Theoretical Computer Science Stack Exchange, I wrote: 


  Expect to spend about an hour per page, mostly on internalizing the paper's results and techniques. Be pleasantly surprised when it doesn't actually take that long. (If it takes significantly less time than that, either the paper is either exceedingly elegant and well-written, you know the area extremely well, or the paper is technically shallow. Don't confuse these three possibilities.)


No, the answer does not depend on whether you get credit for the review.  If you're not going to write a thorough, professional review, just say no.