Reverett, we use those all the time in physiotherapy. Some tips:
1. you don't want the muscle to fatigue too rapidly, so ten seconds on and ten seconds off is a good way, gives the circulation time to recover from all the little capillaries being squeezed by a strong contraction

2. I usually start with 15 mins. and may work up to 30 mins.

3. my stimulator gives a fairly strong tingle, so people often need to slowly work up to the point where the muscle actually begins to contract. It looks sort of flickery or wobbly at first, then becomes firmer. If you give your skin time to accomodate to the electricity, you can gradually turn it up without having too intense a feeling.

4. You'll know it's too strong if your muscle starts to cramp, like a painfully tight feeling

5. some people really notice whether the positive or negative electrode is at a certain end of the muscle, try both ways and see what gives you a better contraction for less discomfort

6. I don't know what kind of electrodes you have. Mine are carbon rubber and you have to use a transmission gel with them, they don't feel good at all without it.

7. they also feel better if they are in firm contact with your skin. I either strap them on, or put a weight on top of them

This may make them sound scary! I think it is sometimes for patients who are in pain, have had surgery, are nervous etc. However, I had it at home once, and my kids and their friends had a hilarious time trying it out, no worries for them!