I'm guessing it wouldn't be a matter of hours - a couple days, or so. Also a couple days after the missile attack. This is based on... nothing at all. I just thought it was a reasonable estimate.
Also, Tir, it has been described in the past that a single Star Destroyer could drain a planets' resources to maintain. A
government, that collects taxes and has many worlds and has many citizens whom it can call upon to crew and build such ships, sure, could probably afford a fleet. Most do. The Coalition has a warfleet of vessels because it has government-run foundaries and government-owned sources of resources. It gets its' crew through enlistment on the grounds of patriotism, public service, ethical cause, or free education - the point is that governments are practically expected to have warships.
Thus, if someone has scrounged together enough ships not just to raid a convoy or kidnap a trade vessel, but to full-out-assault a planet, they must have serious resources, incentives for their men, the works. It's seriously unlikely that two entirely different organizations we've never heard of have launched fleets at the same time to attack us.
It probably isn't the Empire - they use brand-new Star Destroyers, and are generally very proud of declaring who they are and what they're about to do. It also can't be the Black Dragons directly, since they use a lot of their own special ships. We're at war with the Black Dragons, however, and in war people often hire mercenaries or some such thing.
So what happens? Kubindi is under attack, the commander looks out and sees some ships that don't have anything to denote alliegance. He remembers a little while back the same thing happened at Tammar. Thus, he reasons, it's probably the same guys.
Is it really that big a deal? It's not like it'll influence the outcome and it doesn't give us any new information.