When is it?

Act one, scene three.

I believe that the season in which the scene is set would be late autumn to winter. This is because my chosen character of Fräulein Schneider is wearing a dressing gown and slippers. Generally in spring and summer you would cover up with a thinner fabric then a dressing gown. Furthermore whilst settling the guest into the room, Schneider takes a blanket out of the armoire. Once again blankets won't be used in spring or summer. Also, the character of Cliff is wearing an overcoat, 'Cliff removes his overcoat and puts it on the bed.' ((Masteroff J (1955). Cabaret. New York: Tams-witmark Music Library Inc. 43. Page 13) 

The time of the day is difficult to tell. It could be that Schneider had slept in and has woken up in the middle of the day. More then likely it is early morning as other lodgers are taking showers. It could be that Cliff has arrived so early which is why Schneider has been caught in her dressing gown and slippers. The time of year is also one that is tricky to tell, I think that it could be around 1920. This is because the furnishings in the house are minimal. I think this could be because of the sanctions placed upon Germany in the Treaty of Versailles, which saw German people lose possessions. Therefore Schneider wouldn't have had much furniture to give to her guests. 

Berlin winter fashion. 1920. Viewed 18/10.14