I think it's a fair question, personally, and suitable for our Meta since the expectations here are likely different than they would be at some of the more popular sites.
In my experience, it's not normally that much extra work. Those interested in pursuing the moderator position are typically already active in the Review Queues, so there's usually not much workload added in that arena since the user is already doing similar work.
The main time investment is a result of the larger issues: tricky Meta questions, reaching out to individual users, and, in extreme cases, dealing with particularly difficult users (like the instance that Dom and Doktor Mayhem handled beautifully a few years ago).
All in all, for our site—and recognizing that our traffic has declined over the past few years—I'd estimate less than an extra hour each week. But I'd be interested in what Dom and Doktor Mayhem have to say; perhaps my time commitment would have been more had my circumstances been different.
Lastly, I'd be remiss not to mention the moral/philosophical aspect of moderating at Stack Exchange. In light of recent events, many moderators have stepped down, gone on strike, etc. as a form of protest. I think any interested moderator candidate should be aware of SE's recent history to make sure they would be comfortable with being a moderator for this site.