It's hard to be sure, but I think I spend perhaps 15-30 minutes a day on routine moderation stuff stuff like check the flags and act on the easy ones; scan through the days questions (I do this while looking for stuff I want to answer which feeds into my uncertainty about how long the moderating takes) looking for places where the discussion may be going off the rails and casting votes that seem appropriate; pop in to chat to take the pulse of that subset of the community.
None of that is much different from what a high-rep user who is active in the review queues would do except that my votes are binding and I tend to adopt a wait-in-see approach to borderline cases.
But the time spent on less routine things varies enormously. "Less routine stuff" includes investigating the less obvious flags, taking part in the on-going but asynchronous discussions we hold in the moderation chat rooms, and sometimes doing some investigation of hunches (which mostly come up negative, BTW, but have occasionally turned up attacks on the system using many accounts or multiple sites).
Some of that is optional and I simply won't do it on busy weeks; particularly the exploratory investigations. But some less routine stuff needs timely action and I have felt obliged a couple of times when I was very busy in "real" life (say when grading final) to take some time off that pressing work for a PSE moderation issue. It's not common, but it happens.
There isn't a lot of social pressure to be active every day, but the system keeps score of who is handling the most flags and things like that, so you can see if you are slacking or not.
Two changes I noticed when I started moderating
I have to be more judicious in picking things I want to argue for (or against) in public.
Because I speak with a `official' voice, I have to be careful to distinguish my opinions and preferences from site policy and I have to keep in mind that my utterances are going to be scrutinized. I've felt at times that I needed to apologize for things I wrote that I might have just left if speaking without the diamond appended to my name.
I haven't missed visiting the site for a single day since becoming a moderator unless I was stay-in-bed-groaning ill or spent at least six hours traveling.