Siege bug or feature?

[[DnC]]

There is a certain time where within the siege must be succesful (30-45min? Depends on castle type?). However there is an option that turns off the battle time limit and then you have to defeat the enemy!

It can be turned off during a campaign somewhere in the option menu if I'm correct, as well at the start of a campaign there's a box on the right with a textline besides it, where it says "Battle Time Limit" or something.