When did they buy their last tv wall?
When you say "they" I assume that you are talking about Guy and Millie Montag. If so, they bought their last TV wall not very long ago at all. Specifically, they bought it only two months before the time that this conversation is happening.
This is important because it shows how materialistic Millie is and how she is never satisfied with what she has. This is just how their society wants people to be. She has gotten her third wall (at huge expense) just two months before and she is already bored with it and wants another one. She thinks that will make her happy, but it's pretty clear that nothing can truly make her happy because her life is empty.