I our area they were gone by the mid 60's.   I knew people that would leave the cream cans at the railroad depot and train picked them up and took them to Mandan.

Now my father in law in SW Minnesota used them until 1985 when he finally had to put in a bulk tank and get milk picked up.   I still remember how surprised I was the first weekend I was at their farm in 1984, he comes into the house and asks if I want to ride to town.   Back of the pickup is loaded down with milk cans.   Back up to the dock in town, unload the cans, drive around to other side of building, pick up the washed cans and go home.