one of the best things about excercise is that it forces you to pay attention to your body. if you're sleep-deprived, eating crappy food or somehow being bad to your body, you'll find out when you're excercising. how does this relate? well, in my admittedly limited experience, youy body will tell you when you're biking too hard. train as hard as you're comfortable with. as the others said, don't compare to other people, just yourself. other people may have done spin classes, or excercised hard in other areas, or be generally fit without excercise (although i do kinda hate those people
). if you realize you pushed it one day, ease off a little bit. conversely, if you go for a decent ride and your legs are still twitchy, take another 10 minutes!!
also keep in mind that just beginning to bike involves a HUGE learning curve. you're challenging yourself in so many areas all at once (muscular endurance, cardiovascular endurance, road skills/mtb skills, mebtal endurance, and figuring out this new @$% piece of equipment) that every ride is a challenge. so go easy on yourself!! if you beat yourself up about not biking hard enough/fast enough/long enough it becomes just another chore, which sucks all the fun out of it....
I used to dream about ice cream, Antonio Banderas, and daquiris on the beach. Now, i dream about fresh pavement...