Scott,
For some background information on electrolytic conversion of rust, you can read the article I wrote for The Casting Call, Volume 10 Number 3.
As Russell implied, it is the current that is important because it is a measure of how many electrons are being transferred to the rust and it is the electrons that do the "work" for us. The current flow is dependent on several things.
1. Voltage - the higher the voltage, the higher the "pressure" pushing the electrons through the electrolyte solution.
2. Electrolyte concentration - more ions in solution generally means less resistance for the flow of electrons.
3. Geometry - bigger anodes and cathodes will conduct more current, as does moving the pieces closer together.
4. Electrolyte - some ions make more conductive solutions than others
5. Temperature - conductivity increases with temperature
A note about bubbles
Bubbles can remove crusty, old seasoning or rust scales by physically forcing the material off the piece that you are cleaning. The evolution of bubbles has nothing to do with converting iron from the ferric oxidation state (Fe+3, the red rust) to the ferrous state (Fe+2, the black rust that washes off). Electrolytic conversion will occur at less than the voltage required to split water molecules. Bubbles form because the voltage we typically use (12 volts) is high enough to also split water into hydrogen and oxygen, hence the bubbles. This is an inefficiency in the process because after the bubbles physically remove crud on the pan, the current that goes into making the bubbles is not doing anything to further clean the pan.
