However many days it takes for the convention to finish— that’s how long we have to get them out of that tower. Ideally with enough of a margin to get himself entrenched. If the count comes back to a tower under siege, of course the liege rides to the aid of a vassal. If the count arrives to find a new baron instead of the old one… Marco thinks he can make a deal.
As for the neighboring barons, arrangements have already been made with the three closest neighbors. They’d like to save their forces just in case they need to resist being replaced by the count. So the neighbors won’t be intervening in this dispute, staying out unless someone obviously starts winning. It also helps that Marco recruiting every bandit around for this war means the bandits are causing problems here instead of there.
Without siege engines and without time to wait for them to run out of provisions, this can’t be a conventional siege. Either we get the baron out of that tower somehow, or it’s all over.