J.C.
I really think that it should be based on what you are comfortable with. We used a monitor for my oldest until he was 6 and we still use one for my 4 year old. Why worry about whats going on in his room if you don't have to. Eventually you will get used to his noises and wake up when he needs you. I have had many nights where my kids have woke up sick in the middle of the night and if it weren't for the monitor I wouldn't have heard them when I did.