What's the minimum amount of entertainment you'd consider necessary

$40 a month is pretty reasonable for myself.

Would you criticise them for putting a poster on one of the walls?

How much did they spend? $200 absolutely

My issue wouldnt be with them trying to entertain themselves instead of sitting in a totally blank room. My issue was that their entertainment was expensive. Go on a hour walk in the morning. Head to a library, read a book. Both of those are more entertaining than a poster and free.

And yes, all of the decorations in my house are free - found on the side of the road or taken off construction sites. I only spend about 40 dollars a month on entertainment.