16 days in Thailand with GoPro Hero 6 Black [Help]
The important factor is not "battery life" as such. It's mAh (milliamp hours) which is how much current the battery pack can deliver for how long.

For example, my battery pack can supply 20,000mAh to one of my devices, which has a battery size of 4,000mAh. So I can fully charge it 5 times from the pack.

As to your other question - I have had days of 10 hours of footage, and others of under 15 minutes. It will depend entirely on what you do and see, the weather, your preferences etc.