How long can the iPhone 12 Pro Max 256 GB record 4K/4K60fps video continuously before the battery dies?

How long can the iPhone 12 Pro Max 256 GB record 4K/4K60fps video continuously before the battery dies?

I'm in a wheelchair and I want to film my walks in 4K and create a YouTube thing called "Kain the Easy Rider"... before I bought my iPhone Apple assured me that my iPhone 12 Pro Max 256 GB could record 8 hours continuously at 4K/4K60fps on one charge, I thought perfect because I only needed 3-4 hours. Today I did my first test and I could only record 1 hour and 19 minutes and 50 second before my iPhone died.

I'm pretty sure my phone was fully charged so what am I doing wrong or is 1 hour and 19 minutes and 50 second my limit?

Would Aeroplane Mode help if it was on? or and other tips for 3 hours vid on one charge.