My home was treated for termites a year ago. The chemical used was Termidor. Since that time, I have noticed termite activity in the same location on three separate occasions and the area was retreated twice. I noticed more termite activity today so I am very concerned. My pest control company says Termidor takes time to work so they don't seem too concerned about my recent termite activity. The area was spot treated a month ago.

Since termites have been feeding on my home for a year or more, I am very concerned. I have some questions I hope somebody can answer for me.

1. Is there a way to determine if termites are living in a wall?

2. Is there a way to treat termites living inside a wall?

3. How long does it take to apply Termidor correctly to a home? I am concerned that the pest control company didn't apply the chemical correctly since it only took them 4 hours to apply the Termidor to my 2400 sq ft house.

The company may not be mixing it correctly. Hope they aren't charging you for each time they come out.
Termidor should work immediately.
I going to assume you have subteranean termites. Do you see shelter tubes (mud tubes) on any walls? The termites you see should be white in appearance. These are the workier termites. They do the damage. They eat whatever has celluose content in it the take back to the colony and feed the colony.
One thing subteranean termites need in moisture. Check the location where the activity is and see if there is a water problem in that area.
To answer your questions, there is a scope use to look for termites in walls. The pest control company should have one. If you need to treat inside walls, there is a pesticide foam that can be injected into the wall cavities.
And to apply for subteranean termites, they should have drilled holes or trenched around the exterior and inject the termidor. Then on the inside they need to treat also.
You can go to termidor.com to learn more about the product.