That depends on your physical condition and the goals you set for yourself. The joy of STP is that each rider can set their own pace and test their own personal limits. Two-day riders need to be sure to ride two days in a row on several weekends.
Consistent training is the key to building endurance.
Have fun and train with friends with similar riding abilities. Use the Suggested STP Training Miles to help structure your training. It has been said if you can do the 60-mile Flying Wheels loop you will have no problem doing STP in two days. Ride STP with friends who ride at a similar pace. Develop a pace that you can maintain and enjoy, and avoid bicycle burnout.