the time needed to earn karma may be bounded on one system, but across multiple systems it's not.
in part it depends on the karma system
one system I know of won't let you post anything in the dev section until you have posted enough messages outside of that section.
people can post stupid messages to reach the count, or they can wait an unknown amount of time until they have good useful messages to post. how long is this going to take?
requiring that people build up karma eliminates 'drive by' assistance from experts along with 'silly questions' from newbies. If you are doing kernel development (especially reverse engineering odd hardware), wouldn't you just love to get a comment from Linus? A Karma system will prevent this, and I guarantee you that your walled sandbox is not important enough for him to jump through hoops to participate in.
one of the criticisms of contributer agreements is that they raise the bar to contributers by making the process of submitting a quick patch hard. Karma systems do the same thing to block contributions.