You know, take the first wall then the enemy can sit there for a bit and wait 10-15 minutes before bringing and reinforcements.
I think the only chance of this happening, if your 'noob enemy`s' mom calls him out to dinner, and he forgets to resign
When it comes to actual siege combat, things will move at a fast pace in DoF. Don`t expect your typical siege to take more than 5 minutes, unless your enemy has really dug in.
And as far as reinforcement goes, don`t forget the army camp. Almost always when sieging a castle you`ll need to set-up an army camp right before his walls. You can build cheap units like basic archers, and all kinds of siege weapons, as well as heal your troops in your army camp. So you won`t need to worry about time it takes you to reinforce your troops that much.
To counter the point that player inside that Stronghold is done-for, keep in mind that scaling walls creates an obstacle which puts defenders at almost 2 against 1 advantage. Add the fact that players, especially Elves can have a good deal of their resource production inside their town on top of that, but most importantly - the fact that if first wave of attackers fails to get inside - town player will loot all the dead bodies and have enougth resouces to mount a counter attack.
So in a way - yes, sieges would usually take just a few minutes for a massive battle to take place, and either town defenders to get killed, or be in a position where they can ride out and lift the siege.
I have attached a brand new screenshot to show the concept of army camp right by the castle walls. There are two army camps on that screenshot - so imagine - you and your ally about to lay waste to that town.