If you get it at the right stage once would likely be enough. But than again it doesn't hurt to hit them mulitple times. The electric toothbrush is the best tool to use for pollination. Some people even use tunning forks I can't remember the frequency off hand (possibly middle C), but this frequency is supposed to mimic the vibrations of a bummble bees wings. Just stick with the toothbrush though.
A fan can help I also shake my plants when walking by this can help release pollen as well.
Here is a pic of the ideal time, this is also when you would collect pollen when making a cross. A toothbrush would be used for this as well. If you don't have a whole lot of plants I would do it slightly before this stage at this stage and maybe once (or more) after just to make sure. With a lot of plants this can become time consuming that is where the good old shake or lightly flick the flowers can work.
After the petals/sepals have started to curl back the flower is now receptive to pollen.