Exactly because you are gunning for results you are better off to reproduce what recent studies have measured. The following is quoted from this research: Neuromuscular Adaptations to Electrostimulation Resistance Training, Maffiuletti et al. - 2006 American Journal of Physical Medicine & Rehabilitation
the individual was asked to progressively increase the stimulation intensity (range, 0–120 mA) during 5–8 contractions, until a maximal tolerated level (pain threshold).
After 8 weeks there was a force gain of 20%. The study showed that this gain was obtained by stimulating the quadriceps at frequency (50 Hz) used for resistance type 2a fibers, converted the properties of both type 1 fibers and type 2x fibers into type 2a.
Another problem with inexpensive machines is that they do not measure the current intensity. I quote from the same study
The maximal intensity increased proportionally from 48–50 mA (from first to third training sessions) to 75–80 mA (from 16th to 18th sessions).
Therefore whoever does it without reading the mA level is left out on a limb not being able to adjust intensity properly, and having no clue what the proper level is. Again you equate pain with gain, but it’s not true. Doing it properly will give you the gain.