No more than usual. The app can run in the background. I always have it like this!

If the question is still relevant: enter the route and set the display to idle mode (i.e. dark). That's enough. You can stow the mobile somewhere. You don't need it for navigation. It just needs to be within Bluetooth range. The mobile phone battery will easily last a few hours. In addition, you can always call up the charging display of the Beeline and mobile phone via the small round screen...

For a long tour, it is best to start with a charged iPhone.
Belline connected does not in itself increase consumption too much.
However, I recommend keeping the iPhone connected to a powerbank if you do more than three to four hours.