How long can Belgrade hold out?

[seathom]

When I view the supply filter map and look around Belgrade, the city and at least 3-4 surrounding regions in every direction is green signifying my supply can move around but Belgrade should be having to live off itself with no supply support. This has been the case for at least 4 months, if not longer, yet it's supply bar hasn't gone down on the unit card. I became impatient and went O/O on my activated stacks (several stacks in region were, as usual, inactivated) and it did fight at 100% supply. I really want to get my A/H units out of Serbia and over to the Italian border, where fortunately, I've been able to scrap enough units away from Russia to stop the Italian advance.

Is this normal that a stack (PWR was about 750 before the battle) could last this long without getting supply in?

[seathom]

At first, I thought there was some slim chance that supply was being transported through the rivers, but that definitely wasn't the case as I blockaded Belgrade and their stacks were still in supply. I finally did a Ctrl + click and found that Belgrade produces about 115 supply points a turn, which in my estimation is enough to supply that size of a force. Doesn't the CP having about 66% MC lower their output? If it does, I guess they still have enough supply production so I will have to go in there with brute force when I can muster a little bit more force.

[seathom]

I finally was able to dislodge the Serbs from Belgrade in (April '16!). My biggest problem was getting a leader in there with a better Strategic Rating so he'd be active more often to enable a breach in the walls. Inactive leaders cannot breach the defenses (makes sense) and I had poor leaders there. Now I only have to mop up their forces and can finally send my Bulgarians down to finish off the Greeks as the Turks have been slow-going it with very poor transportation system in northern Greece. I should be more daring and send an amphibious force to cut off their supply from behind while I'm at it.