YouTube users now watch more than four billion hours of video through the site each month. For those of you keeping score at home, that's 456,000 years of cat videos, sports highlights, TED Talks and other content per month. Or, the equivalent of watching the original Nyan Cat video about 66 billion times.

First reported as a casual mention in a Wall Street Journal article earlier this week, a YouTube spokesperson confirmed the number to Mashable on Wednesday. And things are moving fast with the video sharing giant — the company just announced in May that users were watching a piddling three billion hours of footage per month. That's the number YouTube still reports on its official press page as of this writing.

That page also reports that YouTube users upload 72 hours of content to the site every minute. In fall of 2008, users were uploading just 10 hours of video each minute.

While YouTube may be best known its viral videos, the Google subsidiary has recently been experimenting with more original and professionally produced channels as a way to attract even more viewers — and ad dollars. Around the beginning of this year, YouTube invested more than $150 million to fund channels featuring stars including Amy Poehler and Wayne Brady. In May, it said it would sink another $200 million into the project.

If the new channels continue to succeed, it may not be long before YouTube sucks us all into watching five billion hours of video through the service each month.

How many hours do you think you spend each month watching YouTube videos? Share with us in the comments.

Image courtesy of iStockphoto, ozgurdonmaz