Good question it depends as I use it on Turbo mode which is the 850 lumens a lot of the time and as the battery packs only a 2200mah one it has limited life on turbo before it drops down to high or low which can be adjusted by a rotary switch on the battery housing.
I don't really know the time (it does state something in the specs) but as I carry multiple batteries as soon as I notice a reduced output I change them .
The ideal ones would be some decent 3400mah ones but at @£20 each it's an expensive upgrade.
I use rechargeable AAs once I've tired the 18650 pack but that's only because I have loads.
You can pick one up off Ebay for @£90 usually @£120+ or wait for a private seller to turn up as happened 2 weeks ago @£60 I nearly got one for a back up .
Some people don't like head torches but you can add the longer lead it comes with and use it as a hand or remote torch also .
depends what your needs are ? A Lenses P7. 2 is enough for most people and interiors of buildings etc even some tunnels if if you do very long exposure which is what I do.
or get one of the Nitecore TM series which is what I've just added to the collection
Lots of batteries is a must for a good headtorch then - my current one is shit but lasts forever- but it is pretty much redundant anyway as it is so rubbish! Think I might go for a senser P7 or similar to use with my decent torch and just like you say go for a long exposure!