How much time should one spend refereeing a paper?
The slightly unsatisfactory answer is "enough time". You should provide a constructive review and that might take time. How much, depends on, for example, the quality of the paper, the complexity of the paper and, of course, your own experience. As new to reviewing, you probably spend longer than what you would with experience. Having said this, I would say that between close to a full workday to maybe two might be expected from an average research article.

Since you are a beginner, I would also recommend that you take your time. A common beginner's "mistake" is to focus on details more than the larger perspective. You need to focus on both. A paper may be poorly written but contain good science (and vice versa) so attending to all perspectives of an article is necessary, and that takes time.

This may all sound like a chore no-one would want to do but reviewing can be a really positive experience since you get to read a paper in such detail as you would probably not do otherwise, you see new science and you get to contribute to science with your expertise in ways other than producing your own papers.