When I first started teaching centers and even when I moved to a guided reading approach my biggest issue was student stamina. Starting off the first day of school building reading stamina and charting our progress was a huge success.
My 2nd graders were very eager to see the bars on the graph increase. When student stamina increased significantly I start bragging to other teachers, the principal etc. Students love it when you brag about them about their progress. Students even love to use the word "stamina".
Some tips: I usually start at 3 min. but this will vary depending on your class and grade. If I see that students might be able to go another minute or two on the first day I let them read longer. The goal is to have students feel success right from the very beginning, especially those students who haven't had tons of success in the past. This gives students motivation to go even further on the next attempt.
For the most part I do take things slowly. If I have students getting up or start talking I stop. I am very strict at the beginning of the year with this because I want students to reap the rewards later.
Here are the stamina graphs that I use.