To be fair, that's kind if your fault for ignoring it for so long.
There's absolutely no reason for you to wait 800 days before assembling the banner. It doesn't change the gameplay at all or force you to start claiming land or building your own kingdom. You're intended to assemble it before you even become a vassal to someone. (either to give it to them if you intend to be loyal, or to keep it for yourself for when you finally start your own kingdom)
That said, I can't see any reason why there would be a time-limit.
Did all of the imperial factions get wiped out or something? That might explain why it failed.
Either way, if you start a new game, just assemble the banner as soon as you're done with the tedius 'talk to all the nobles' quest, then you can put to aside to play vassal or whatever.