Same goes for me. I'm suspecting some form of ADD (but I'm highly functional, so no need for diagnosis, it does runs in the family, other family members are less lucky.)
I can spend a large part of my day here running/hiding from the ultra boring parts of my work.
I imagine my karma comes both from submissions and comments. Some submissions have 100+ karma, some comments can go to ~40 karma. Most are 1-2 karma, some 5-15 (take this with pinch of salt, I didn't really analyze it, it's a guess).
Always a fun time when you get flagged and personally reprimanded by dang but your comment is tremendously upvoted.
My posts mentioning Ayn Rand are also heavily fluctuating, sometimes tipping up, sometimes tipping down.
I always wonder: What are the most up AND down voted comments? I mean you can score a 1 with 0 votes, or with 100 up and 100 down (I guess, not sure if that is the formula). The second remark is of a different nature but will not give different karma.
FWIW, In all cases I agreed with Dang. I get carried away "twitter style" sometimes. Dang guards the quality (no short snide flaming remarks) and focus (we talk tech here people) of HN threads well.
For comments I find that a polarizing comment may yield karma but tip one way or the other (you know, it will polarize), but finding a smart middle ground though, is quite a sure way to score karma.
Most of my Karma comes from really random comments that I assume people will not like to hear. The overwhelming majority of my comments will never get more than one upvote (if it even gets that).
The comments that earn karma are really 50/50 between if I get flagged or suddenly get 50 upvotes.
Regardless, you shouldn't ever expect to get any upvotes, karma doesn't matter except to add some amount of merit that someone is not a troll. Outside of that it's quite useless.
Answering the question, it ranges from 15 minutes to 4hours a day.