From an admin point of view, priority for me is usually checking the moderator forum for any issues that have arisen that I need to deal with.
At the same time, checking recent posts since last visit for spam, etc is next on the list (usually mods do a great job covering this tho! ;)).
Week to week (or perhaps monthly) a review of the general subjects in each forum can be good to give you ideas for new forums to add, etc. While you are doing this you can also move topics to other forums if need be (if you aren't doing this more regularly).
Most forum software has great community support, so it's worthwhile keeping up to date on their forums - new add-ons, updates, etc.
I started off as admin by replying to everyone who posted, but after I while I felt that this may deter others from replying, so I tend to leave posts to let others deal with them now and only reply if there hasnt been any response.
It's good to take a break from the forums - being to passionate about them an just drain your time from other duties! But you really need others to rely on in your absence (moderators).