My daughter (now 9) was bald until she was 2 so thinking about when to get her hair cut was never an option.
When her hair did start to grow, she was "white blond" when it came through although she is now "dirty blond", but I think she looked more bald than what she really was because her hair was so far.
I started getting it trimmed when it got "wispy" - she has never had more than 1 inch off and it is now down to her bottom and she will not consider having it cut short although I know, one day, most probably when she is a teenager, she will have it cut short and then spend the rest of her life trying to grow it again!
My son had lots of hair from a young age and had his first proper hair cut at 6 months. He had a "No. 2" once but, as Jodee says, it did make him look like a thug. He now has to have his hair cut about every 4 weeks - he has (what I think is called) a flat top - to be like his dad! Whereas my DD is blond, my DS is darker. DD has blond eyelashes, eyebrows, etc. but DS has very prominent eyebrows and the longest and thickest eyelashes you have ever seen - why is it always the boys that get them?
My baby is nearly 5 months old and I think she is going to be like her elder sister. Although she was quite dark to start with, she is now a baldy and has very fair eyelashes and eyebrows so I think her hair is going to take a long time to come through like her sister.
One thing I would say in my two elder children's defence is that, although my son was very young when he first had his hair cut, we have no problem with him having it cut. Saying that, although my daughter was over 2 by the time she first had her hair cut, she has always been very good too. Because her hair is so long, as soon as she started growing it, I was very particular about it being brushed, etc. I would stand for no nonsense, and although it takes quite a while to brush, plait, etc. she is very good. She knows that if it isn't brushed, I would try and make her get it cut so she has to put up with the conditioning, treatments, etc. to keep it looking nice.