As much as I can comfortably carry. I grew up in the woods, been there all my life and am comfortable with a warm coat and a pocket knife. However, the unexpected happens. If I fall and break my other leg nobody I'm with is going to carry my hefty butt out, so I might want to plan for such things. A day hike for me can put me miles away from road or house. At a minimum I am carrying water, pistol, and a pocket knife. I usually take a camel-bak mule which provides me with 100oz of water, 2 more knives, esbit tabs, firestarter, spare mags, 550 cord, trash bags, dry socks, first aid supplies, flashlight, strobe light. chem light, whistle, sun screen, bug repellent, duct tape, etc. Add a mess kit, a little food, field jacket, and a blanket and I am set to hunt for several days alone in cold weather, did it last year, broken leg and all. True, I added a rifle and upgraded the pocket knife to a khuk,
but all my crap was in the camelbak and jacket. I see no reason to assume all will go well on a day hike, I am reminded of all the famous last words in history; like the Rangers in Somalia who didn't need nightvision or water because it was a quick day mission. Better to have and not need than need and not have.