I'd say that a new hubber would spend more time on HubPages a day because there are so many pieces they need to read in the learning center, not to mention just generally learning the ropes, researching and writing an article.
That said, I'm betting that new users actually, for the most part, don't put forth a huge amount of effort. I know that for me, I essentially just slapped my first articles into hubs without pretty formatting and whatnot.
It wasn't until I became an HP addict that I really started putting time and effort into my hubs. That said, I put forth more time per day into HubPages than I used to. However, I don't have the learning curve to deal with anymore so I really could cut down the time I'm logged in if I really wanted to.