Would aim to use it on 600k rides and LEL, so would definitely need to charge via dynamo hub on the way. Hopefully it's a bit less quirky doing this than garmins. Feedback online seems so much more positive than Garmins, but have struggled to find much about longer distances riding with them.
I've used an Elemnt for up to 12 hours (including stopping for lunch etc), using the navigation. That's about as long as a full charge will last.
I've not tried to charge it up on the move as yet.