I started biking this year on April 2nd. (Ignore my join date, I joined a while ago, but didn't buy a bike for long after that.
My first ride was 3.6 miles. I probably could have gone further but I was sore. I did 15 miles later that week, which I thought was tough, so that should give you an idea of my starting fitness.
I've now been biking for less than 2 months, and in my opinion, I can ride 40 miles, so I think the changes come pretty quickly, at least early on. My longest ride to date is 32 miles, but I felt good afterwards. I'll be biking 45 miles on June 9th, so I guess I'll find out if I can do it or not then.
I think the biggest change in time is the ability to recover. When I started, If I did 10 miles, I needed a day off. I don't mind going 20 miles back to back on consecutive days now.