How is MJ distributed?

Buildcraft energy is distributed by requests. A machine will request an amount of energy from the network it is attached to, and if there is power to meet or partially meet that demand, it will be delivered, subject to any losses along the way.

The answer to your question is complex, and depends hugely on what machines you have connected, and how they go about requesting energy, and how much they can store.

You can actually work this out for yourself in game using an MJ Reader to see how much power is, on average, arriving at certain machines.

However, as you can see from the description of the tool, it is a complex process, requiring averaging measurements out over a period of time. There is no simple answer to this.