How long does it take for treatment with tens units to start working?

Varies: Tens (transcutaneous electrical stimulation) is a non-invasive modality to decrease pain perception in many body areas. The concept is similar to when someone hits a body part and then immediately starts rubbing the area. The latter sends a lot of non-painful stimulation to the brain and essentially pain impulses are drown out. The effect is rather immediate and wears off quickly once it's off.

TENS Time: Tens, or transcutaneous electrical nerve stimulation is a therapy used to "exhaust" nerves and deplete them of neurotransmitter to limit the transduction of nervous impulses, or pain. Depending on the location of the problem on the body, the electrode size, the impedence or resistance of the skin and deeper fatty tissues and voltage of the unit, the amount of relief can vary; adjust with caution.

TENS excellent but..: Tens (transcutaneous electrical neural stimulation) can be very effective if used correctly and in conjunction with other exercises and therapy. Please consider a neuromuscular TMJ specialist. I use tens often, but not as my only modality. It can take time, but is an effective measure, again, in conjunction with other treatment.