I ride kilometres ...
Some months, like this one, I hardly ride at all. It's late autumn, it's dark when I get home from work, and it's been quite chilly so I'm taking a break after all the summer riding.
Some months, I ride 1000 km. I like to do that at least one month a year. During the summer, if I don't ride 1000 km in a month, it'll be close to it (850 km, 900 km, etc.)
But my distance riding has really backed off from what I was doing between 2001 and 2005. In that time period, 1600 km months weren't unusual.
Over the past 18 months, Rowan and I have been doing a Century-A-Month challenge where we do at least one 160+ km ride each month. I did the same thing each month in 2003, 2005, and 2006, and completed 11 months 2008. If you did that, right there you get 100 miles in a month ... you could just about double your current distance.