My 800 would max out at 13h with minimal nav and I never connect anything like Wifi or BT.
810 has the same battery I believe so you're probably looking at 8-10hr mark for what you're doing but it all depends on temp, age of battery, etc.
I normally use my 800 for time trials (with a Garmin battery pack for 24hrs).
It only differs in it's a torpedo shape with strap to fit bars, rather than a box like most battery packs. It's just enough for 24hr with no navigation. You can normally use any external pack so long as the pins don't make it go to USB mode.
My 800 would max out at 13h with minimal nav and I never connect anything like Wifi or BT.
810 has the same battery I believe so you're probably looking at 8-10hr mark for what you're doing but it all depends on temp, age of battery, etc.
I normally use my 800 for time trials (with a Garmin battery pack for 24hrs).