Some people like to increase by paying attention to time, some attention to mileage, etc. When I first started out, I could only do about five miles before I wasn't having fun. I increased by adding 10 minutes to my ride whenever I got out there. Very quickly, I started increasing by mileage, by adding two mile more in a ride, then five, then ten, etc. because I really enjoyed the challenge. Find something that makes sense and works for you, but if it's not fun, it's going to become a chore quickly. I'm now coming back from an injury, and about three months ago, doing about 10-15 miles was all I could handle without getting irritable and moody, so that's what I did. I increased based on my mood. Two weekends ago, I did 120 miles because I was enjoying myself. Last weekend, I just didn't feel like it, so I didn't ride at all on Saturday, and rode about 30 on Sunday. I can't wait to get out and ride this weekend, and have three rides planned with friends.
I agree with the above posters. Have fun. Listen to your body (mood included). Push yourself little by little in whatever way makes sense to you, but don't turn it into a chore. Congrats on getting a bike and getting out there. Enjoy!
2013 Kirk Frameworks JK Special/Selle Anatomica
2012 Gunnar Sport/Brooks B17
2001 Calfee Tetra Pro/Selle Anatomica
1984 Raleigh Sport/Brooks B66