Franklin Canyon reservoir is north of LA in the mountains, they would have had to clear a site (bulldozer?), truck everything up there, union crew, construction equipment, materials, scaffolding and likely a small crane. One day (maybe two) for construction.
Then the production crew and actors would show up, if they lined it all up, single day shooting on location.
And when the shoot was over, I would imagine their permit would require everything be removed again, although being the sixties the site probably wouldn't have had to be restore to it's original state, like it would today.