Geralt estimated the time through his experience.
To fight a battle, even if it's one limited to a few hundred people, there's a lot to consider.
Take the White Rose Knights who are secretly crafting siege weapons in this hill, for instance. If the weapons are secretly crafted, it means the crafting site must be inconspicuous.
Inconspicuous means the roads are rugged; how could one move such heavy siege weapons without transporting them on properly maintained roads?
There's no way to directly approach High Hill Castle from the wild countryside.
That's why, in those war stories, both sides keep talking about 'this crucial path' or 'this vital point'.
Because when the army moves, counting the transportation of weapons and supplies, the requirement for roads is an evident hard condition.
Unless a Great Mage suddenly appears who can move mountains and fill seas, then how many roads are available, everybody pretty much knows.
