It definitely depends. If we're talking about standardized interviews, in Q&A format, it's not too much work. If we're talking 10,000+-word, deep, New Yorker- or Atlantic-length articles, it's a lot of work. One of those every month is an aggressive cadence for anyone not doing this full time. Writing in-depth stories like those is very research-intensive, travel-dependent, and analytically taxing.
I would guess (?) that they're aiming for a medium in between those two extremes. But even still, bandwidth can be an issue. If you're doing primary discovery, primary research, and primary analysis, you're going to have to commit significant effort to produce something really good every month.
In general, we as readers tend to underestimate (and undervalue) the effort involved in primary research and investigation. Some of what we read in feature-length magazine articles involved hundreds of hours of primary research or experience.