It depends.
If I am at my PC all day, which I have been recently, then I would say around 5 minutes every hour.
If I am not going to be at my PC all day (so most of the time) I would try to spend at least 10 minutes in the morning, and 10 in the evening.
In case you're interested, I have around 1000 individual hits a day these days, most of which are from the UK. It was around 400/500 before I got kotaku'd. I would say I earn £100-£200 a month, but that really varies depending on what deals are around, like when the Rock Band bombshell hit I earned £200+ in 24 hours, but I can go a week and earn less than a tenner.
My advice -
Get a facebook, Digg, Stumbleupon and twitter account. Also get a seperate email address for just the blog, and encourage people to email you (and once you have their email addresses, add them as friends on facebook/Digg/Stumbleupon/twitter)
If you ever feel strongly about an issue, and capable of writing and interesting post about it, do it.
This post of mine got me a lot of traffic.
Edit: 'sup Mr. Grinch!