Agree with the majority of the other posts...it's a very personal thing. Plus, it varies with time of year. In the winter in NJ, 30-35 is fine for me. In the summer, I want to do at least 50. My only suggestion would be to try and increase your mileage (either your longest single ride or weekly total) about 10% each week. That's a very reasonable amount that won't add too much stress to your body but will keep you progressing. At some point you'll find your happy spot (25, 40, 50, 100, more?) and stay there. Might also help to join a club and ride with a group.