Will my iPhone 6 record for over 8 hours straight?

I plan on moving from the upper Midwest to El Paso Texas. I will drive from my current residence to Denver Colorado which is a 8 hour drive. After spending the night, I will then take I-25 from Denver all the way down to El Paso. That's an additional 9 hours ish. I want to take a continues dash cam video of my travels. Will my iPhone 6 be able to handle that? If not what device would work?

It can do that, just make sure that you regularly charge it or keep it on charge as that will drain the battery fast. Also make sure you have enough space for it, recording at 1080p at 30 or 60 fps will take up a huge amount of space.

So you want 17 hrs altogether.

720p video takes up about 3 GB per hour so you'll need over 50GB free space. It will take a VERY long time to download that much video and will you have any means of editing it?

Look at in-car cameras or dash cams as an alternative, and use a 64GB card if possible. A quick check of ones in the UK show some only take 32GB cards.

Assuming you keep the phone plugged into your car's power source the whole time, you'll only be limited by the memory available in your phone. At 720p 30fps (good enough for dash cam) you'll need about 60-80MB/minute, which means for 8 hours, you'll need about 35-40 GB of storage free. I hope you have a 128 GB iPhone, because otherwise you probably won't have enough room for even one leg of your trip. Even with a 128 GB phone, you'll need to copy off the first day's video to a laptop or other storage to free up memory for the second day.

Also keep in mind that an iPhone was never designed to record video for such a long period of time. Heat build-up is the potential problem in that it could damage your phone.