I have replaced -1 with 0 in the second field of message count and entered a time period till one year ago. I wanted to confirm because the number I am getting through this search is pretty large. It is probably accurate as I suppose a large number of accounts are created by spammers and bots which never are confirmed or used or are marked as spam.
I would take a backup of the database of course before running the pruning command.
PS: XenForo should provide a better date picker. I imagine browsers now provide a native date picker using HTML5 tags (Looks like Firefox does not support them right now?). Right now I cannot change the year from the popup calendar so to go back to 2003, I have to press the back button insane number of times. I assume entering the value manually in these fields in proper format would work as well?