Using the OnePlus 6 with built in 64GB storage as a reference. After installing basic apps and downloading offline maps, caching user data, capturing few days of photos, videos etc. one has hardly 40-50% free space remaining.
Using default settings the average photo comes to around 5MB and 30 sec HD video to 150MB. On a typical day one might take 150 photos (750MB) and 5 min of video (750MB). or 1.5GB per day. To capture a journey one requires approx 45GB per month.
That's in HD. If you start shooting 4K video the storage requirement goes up dramatically.
As the OnePlus 6 hardly has around 40GB free space one has to carry additional storage to capture longer journeys. This phone model does not have a built-in memory slot.
One can carry multiple flash drives (32GB, 64GB, ...) and use a OTG convertor to periodically move photos and videos from the phone to an external drive.