How long you can leave it loaded is a hard question. Rusting wise, it depends on whether you shoot it, and what powder you shoot. But lets just say, you load it opening morning on a clean barrel. Technically if all things in the universe are in alignment, and the great hunting fairy is watching over you, you can leave that rifle loaded with no fear of rusting. BUT.....
Sometimes the reason to unload is not a rusting problem but more of a trusting problem. In other words, you did not shoot it. But you have to ask yourself... did the powder in any way get compromised? If it did, lets say tomorrow when the huge buck walks out 30 feet in front of you, will that gun go off. Everyone thinks, inline rifle, 209 primer, the gun will fire. Well 90% of the time leaving the gun like that, they will. But I have found I am the 10% type. I have had two fail to fire the next day. When I trust the rifle will fire when I need it... it goes poof, and the deer giggles and runs off.
So if you covered the muzzle with a muzzle mitt and hunted in rain or snow... be sure at the end of the day to take that mitt off. And wipe the barrel with an oiled cloth. Muzzle mitts will rust your barrel under them. Next, do you think there was any chance the load was contaminated with moisture? If you do, shoot the load off, if it will fire, clean the gun and start fresh in the morning.
If its an inline, you can take the breech plug out, and push the load out the back of the rifle at the end of the day. Don't use that powder again, but if your shots are close, I have tested pushed bullets and found then still as accurate as an non-pushed bullet. Or save the bullets and powder for a range session in the future and load all new.
Certain powders rust faster then others. We all know that. But where we hunt and how we hunt and store our rifle can make a huge difference. Some people are just lucky. They can basically ignore all the "rules of how to store a rifle" and the thing still fires. In my case, I play by the rules and have had 2 ftf. So at the end of the day, I fire my rifle off. I clean it. And load it fresh.