Changing needles?

I just bought a used blendtone machine from ebay and the seller sent me a few new disposable needles with it. I will be using the machine on nobody but myself. After how many insertions do I need to change the needle?

Unless the probe/needle breaks, or the coating flakes, you can use the probe for your entire session and or day. Just don’t reuse the probe from yesterday. As long as you are using it, you are ok. Most people don’t use them for longer than an hour or 3 anyway.