Hello,
I believe most headlamps run on single 18650 batteries or smaller, so it will be challenging to find any lamp that can sustain 500 lumens for that amount of time without stepping down.
You might want to consider headlamps that have battery packs that are separated from the headstrap. These headlamps usually feature larger capacity batteries and designed to work well in cold weather.
Here are a few examples that might work for you:
Fenix HP30R (review below shows it can sustain 400/500 lumens for over 5 hours, and with better batteries I am sure this can be even longer)
http://www.fenixlight.com/ProductMore.aspx?id=2247&tid=13&cid=2#
Acebeam H30
(XHP70 LED that is crazy bright, but this also means it can sustain lower output levels in the 500 lumen range both efficiently and without overheating)
https://budgetlightforum.com/t/-/53693
Due to the larger LED in a small reflector, light will be very floody and will appear less bright at longer distances.
Custom H30 can be offered with High CRI at great price:
https://skylumen.com/products/acebeam-h30vn-brightest-headlight?\_pos=11&\_sid=4787e65b6&\_ss=r
Lupine (VERY EXPENSIVE)
Silva Headlamps (Can be very expensive depending on model)