The actors' contracts end after S6. Kat at least said earlier this year that she plans to quit after S6 and focus on her music career. Also, when they were signing the actors for S7, one of the three (probably Nina) apparently said they weren't sure if they were going to sign for S7. Since then, they've started to say the series might end after S6. There's a theory that it's just a publicity stunt to raise ratings, though.