Regarding short battle time limits:

First I totally agree that time is short. In many occasions I ran out of time and left in frustration.

I guess it is good to allow more time to larger castles. For wooden forts 20 minute is reasonable, for 30-35 minutes for stone castle, 40+ for epic stone walls, etc.

Switching the timer off is quite bad, though. There was a time I turned the battle timer off after a frustrating assault, and it made everything so easy - I feel like an idiot commander. I can issue one order every 1 minute or so, watching 1 unit of archer march foward and shoot, the back off, then the next... it feels even more unreal. I think it is even more devastating than the frustration of timer running low - this makes me think my brain is running low.

So I would recommend not to turn the battle timer off, instead try to fight with timer on and see how much you can improve. Timer off is like a tavern, making you "unsharp" (-1 command, -1 management, -1 influence).