My girlfriend is a squirter (actually a gusher), and one of those who can do it with ease. The first time it happened, we were messing around and she was really turned on. A little g-spot stimulation and clit at the same time and then all of a sudden there was a "whoosh" noise and she soaked me and the bed. Her whole body was shaking/quivering and she could barely lift up her head to see what she had done.
At first she was afraid that it was urine, or that some of it was, but it was definitely female ejaculation. We tried again on a different day, and made sure she had completely emptied her bladder before we played. She gushed again. An amount that would rival Cytherea.
Now that she knows what causes it, and she's more familiar with her muscles down there, she can control it. The poster who said the longer you go, or the more turned on she gets, the more she gushes is absolutely right. Also, she can squirt multiple times. Again and again and again and again. She says she feels something 'filling up' with fluid, and then at will she can let it go, even when just working the clit as long as her g-spot is already swollen.
We were on a 2 and 1/2 hour car ride home one night and I had one hand on the wheel and one hand on her. She had her legs up on the dash and was sort of leaning back a little. She'd fill up and then put my hand in front of her and say "Wanna feel?" and then let go a little. My hand was like I dunked it in a bucket of water. A couple of minutes later, we'd keep the stimulation going, she'd refill and do it again. She must've squirted 8 times on the ride home. After a while, she gets tired there, and it gets harder to squirt, but it's an amazing thing.
I had been with another girl who squirted one time in the whole time we were together, and it was almost textbook g-spot work and it took forever. This was after she was loosened up with a few margaritas. When she squirted it was like two little squirts from a kids 99 cent squirt gun, and I was like "that's it?"
Comparatively, it's like my current girlfriend has a mini-super soaker. And it's absolutely amazing.