How long does a newly risen zombie army wait before moving?

I’m heading towards a newly risen zombie army, which I can’t defeat with the small army i have heading towards it. But, if it starts moving towards the nearest settlement (in the opposite direction) then I can get in and clear the rest of the gravesite. But, it’s just been sitting there for hours. Is there a set amount of time before a risen zombie army starts moving? Or is it random?

My not so scientifically studied answer is that it is random as to when they move. However, the direction they choose is “dumb” in that they always head towards the closest non-blighted settlement, and if that is a tie then it is random between the options. They also don’t optimize to going on the roads vs. going through mountains/swamps/etc.

Thanks. It seemed random to me. I guess it’s random within a fixed minimum and maximum.