It takes about 3 months to get into cycling shape, after which you should be able to ride all day. Most riders limit their riding by the availability of leisure time.
To begin with, keep your rides under 1hrs duration, and dont worry too much about speed or distance. You will be conditioning your butt to being on a saddle. Make sure the bike fits you well, and feels comfortable. After a week or 2, you can think about riding for longer periods of time, 2-3 hours if you have the time.
I always ride by time, not distance. A headwind or hill will slow you down, but 30mins of riding is still 30mins.