When acupuncturists treat muscle tension, how deep do they go?

Depends. That depends on location & technique being used. A needle introduced through the leg or arm could be as long as 75 mm, in the buttocks it could be as long as 125 mm. Often needles are introduced only into the dermal layer as it will still impact the underlying fascia and muscle. These might be inserted a few mm. Depth of insertion for major acupuncture pts is specified & measured in cuns.
Varies on location. It depends on the muscle and it reference zone. The small needle is advanced according to the meridians of chinese acupuncture. These include the 12 main meridians plus their inner pathways and the 8 extra meridians. Usually no more than 1-2 inches, could be less or more depending on the amount of tissue over the site you are aiming for.