What's involved in reviewing an article?
Reviewing peer articles is part of a researcher's job, it's good you get to review an article, the fact you built your master's thesis on it probably means you can have a relevant opinion.

A review typically takes half a day in my experience, could be more depending on how familiar you are with the material, and you were probably given this review by a senior colleague that is part of the PC and will read your review before it goes to the actual journal. 

Just read it slowly, note your "small comments" on the fly (typos, language etc separately) and then try to give an overall view of whether you liked the paper (e.g. an easy way is to make a small 2/3 line summary of each section as you understood it) and the gist of it as you see it. A review isn't much more than that. Being constructive in criticisms is a good practice

So, imho this is part of the job, and you should take the time to do it. It is experience building to review other people's papers.