Mixed Unit Hunting.

I like having 18 hunts because then I have 18 chances at getting stones

You don't need mega units to hunt 15+ stones (2000 inf will do if lucky) so I hope for good hunts and usually pull in 1-2 stone hunts when I harvest all of the troops

Of course, this was all before the map crashed xD

for how long do u hunt to get the stones?

Whatever floats my boat.

I haven't noticed a good time that works "best". I've gone 2 hours and gone as long as 18 hours.

I suppose a shorter time would be better, since then you can get 36 hunts of 4 hours or 18 hunts of 8 hours, and you would likely get more stones with the doubled hunting party, but I just harvest when I log on and hunt when I leave, not really paying attention to how long it was (just making sure I didn't starve)