I have peroneal tendonitis and everytime I try to run or hike it hurts like hell. Someone suggested using kt tape. Does this work?

No studies support it.  it has a cult following because it's cool and the placebo is strong... some of my athletes swear by it.

I had some nasty elbow tendonitis last year (Tennis Elbow).  My PT did some taping.  She was trained in doing it.  She used a reference book for clinicians to determine how to tape my arm.    The way she cut the tape looked like fucking origami.  But I think it helped provide some support/relief.  But that was in addition to the stretching/mobility/pain meds/etc. I was doing.  It was just one more modality to treat the issue.

I think you need someone who knows WTF they doing with that tape vs. someone who was a trainer in high school or college.  I think taping it WRONG could be worse than not taping at all.

My .02

YMMV

How long does the tape stay on?  Based on your post it seems like it would be a nightmare to try and tape everytime you go to the gym.

Depending on where it is (sweat,  etc), couple days to a week (or more)

i was skeptical when i saw people using is..

one crossfit comp i went to my knee started aching a lot after one of the workouts.

i bough some tape and read the little booklet that came with it to know how to apply the tape to my knee...

made a huge difference in how it felt.. so much better.

try it out. youll only be out a few bucks anyways.

i only use it if im getting aches and pains.

Do you know the insertion, origin, and lymph locations for that area?

Ask me about dry shaving my arm at the PT clinic!  

...and I'm a fucking hairy ape!  LOL  

Once you have properly prepped the area to be taped....i.e. shave in the shower...  The tape would stay on for several days that my PT would use.  If it started to peel at the edge I would just trim it.  Depending on condition, she would re-tape as needed.  Generally 4-7 days, but I think it will also depend on the area being taped and how much ROM you have for the area.

That was my second thought...the hair  

I've been having a bit of tenderness in my left shoulder when I "chicken wing".  It hurt today during bench, but amazingly feels great after I did some face pulls and pullups.  I'm pretty sure I sleep on it wrong as I stretch out my arm under the pillow and sleep the pillow and my arm.

One of the guys I work with is a first rule crossfitter and he's been telling me to tape.