I was instructed to use it in 15-minute periods (never said how much
rest time between uses).

Your case may be different, and you haven't really told us anything
about what your original problem is - what type of arthritis (of the
more than 100 forms), your age, etc. etc.

... Firemen still make housecalls.

I have used one with great success for a long time. I wouldn't
leave it on for an hour though, and try the various settings on
low power.

I was told by Physical therapy that prolonged use of a TENS unit will
begin to break down the muscle tissue that doesn't rebuild.  It's okay
for short term use once in a while but I certainly wouldn't use it daily.

I use mine for muscle spasms that cause severe back pain and I sometimes
keep it on all day. I have slept with it on, but usually put the timer on 30
minutes when I sleep with it.

Once, I had worn it at night and took it off in the morning. Later that day,
I put it back on and after awhile, it seemed to be losing it's effect. I
checked the settings, turned it up all the way, and nothing happened. Then I
opened the cover to look at the other settings, then realized it was still
set to 30 minutes and it had just timed itself off. Oh, I thought, that's
all it is. So, I turned it to 'continuos', forgetting that I had it turned
ALL THE WAY UP! That thing bought knocked me outta my recliner! Kind of like
someone had put the paddles to me, only on my back instead of my chest. The
fastest way I knew to get the thing turned off was to jerk the plugs out.
Took me about one/half of a nanosecond . Other than my stupidity that day,
I've had no problems with mine. Using it has kept me out of physical therapy
for months. When I feel the back pain beginning, I put the tens unit on and
wear it all day as I go about my regular activities. Without it, I'd be in
for a 2-3 week spell of misery.

Somebody please tell me why you're not supposed to use it except for such
short periods. What kind of damage is it supposed to do? How? and why? My PT
told me I could use mine as much as I wanted to. Have I been ill-advised?

Gloria