"It took us three hours to come to a conclusion."
Translation:Nos tomó tres horas llegar a una conclusión.
is this grammatical and does it mean essentially the same thing: "Nos pasaron tres horas para llegar a una conclusion"?
You cannot say it this way. "Nos pasaron" means "they passed us". If you wanted to speak about the passage of time, you would, "Pasaron tres horas antes de llegar a una conclusion" = Three hours passed before reaching a conclusion. But the passage of time is not the point of this statement, it is how long it took us..
Although it basically means the same, it's not a translation. "Tomamos tres horas..." means "We took three hours..." rather than "It took us three hours..."
The English uses the impersonal form, so you need to do the same in Spanish.
I think the translation is fine as is, but it strikes me as a slightly Americanized. Would "Nos tardó tres horas llegar a una conclusion" be a more appropriate?
I would use "tardar", as you say, but the correct way to say it would be: "(Nosotros) Tardamos tres horas en llegar a una conclusión" ;]
I would probably say " Nos demoramos 3 horas ...." but it isn't as exact a translation. In Spanish, it is commonly used "llegar a una conclusion" not "alcanzar". So you arrive at your conclusion, you don't reach it. "Alcanzar" might have a more physical implication.