yes I think you are right. If you use "W" answering when? then anything longer than a day seems to be locative ( I guess a night is longer than a day), and a day is accusative,
Also we have w tydzień/w dzień/ w noc / w minutę (accusative) to describe amount of time. (Wybudowali to w dwa lata -they have buid it in two years- it took him two years to build it)
Keep in mind we still have "o", "na" "za" and instrumental
or even beer
The history of small beer dates back for centuries
. In the days when sanitation was bad and water dangerous to drink,
small beer was served to servants, field workers, the poor, even the young
. The first runnings from a brewer's mash would go to a stronger beer, the second for ordinary beer.
A small beer, taken from a third running, was probably about 2.5% alcohol by volume.
Belgian monasteries, in particular, produced large quantities of small beer in the Middle Ages.