How often can I boost with a perfect power distributor?

Approximately once every 5 seconds. In the particular case of my build, this meant that a 4A Power Distributor with Engine Focused was adequate, giving me a boost time of 0:04, however, you may have to experiment a bit for your own build. Access to Jameson Memorial helps out quite a bit with this experimentation process.

Another thing to keep in mind is that reducing weight by dropping a size class has diminishing returns. Going from a 7A to a 6A saves 40T of weight and adds around 4LY of range to my build, but going from a 4A to a 3A only saves 5T of weight and would only add around 0.5LY. Is it possible that the 3A with engine focused and an experimental effect is good enough? Maybe. Is it worth engineering that for a measly 0.5LY, when my range is already 62LY? No.